diff --git a/CHANGELOG.md b/CHANGELOG.md
index e575e8a2a7..b266875df8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,13 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com)
 and this project adheres to [Semantic Versioning](https://semver.org).
 
+# TBD - 2.3.8
+
+### Fixed
+
+- Xls writer Parser Mishandling True/False Argument. Backport of [PR #4333](https://github.com/PHPOffice/PhpSpreadsheet/pull/4333)
+- Xls writer Parser Parse By Character Not Byte. Backport of [PR #4344](https://github.com/PHPOffice/PhpSpreadsheet/pull/4344)
+
 # 2025-01-26 - 2.3.7
 
 ### Fixed
diff --git a/src/PhpSpreadsheet/Writer/Xls/Parser.php b/src/PhpSpreadsheet/Writer/Xls/Parser.php
index c0f60d66e1..dd49b619b4 100644
--- a/src/PhpSpreadsheet/Writer/Xls/Parser.php
+++ b/src/PhpSpreadsheet/Writer/Xls/Parser.php
@@ -67,6 +67,8 @@ class Parser
         . '[$]?[A-Ia-i]?[A-Za-z][$]?(\\d+)'
         . '$~u';
 
+    private const UTF8 = 'UTF-8';
+
     /**
      * The index of the character we are currently looking at.
      */
@@ -991,29 +993,30 @@ private function advance(): void
     {
         $token = '';
         $i = $this->currentCharacter;
-        $formula_length = strlen($this->formula);
+        $formula = mb_str_split($this->formula, 1, self::UTF8);
+        $formula_length = count($formula);
         // eat up white spaces
         if ($i < $formula_length) {
-            while ($this->formula[$i] == ' ') {
+            while ($formula[$i] === ' ') {
                 ++$i;
             }
 
             if ($i < ($formula_length - 1)) {
-                $this->lookAhead = $this->formula[$i + 1];
+                $this->lookAhead = $formula[$i + 1];
             }
             $token = '';
         }
 
         while ($i < $formula_length) {
-            $token .= $this->formula[$i];
+            $token .= $formula[$i];
 
             if ($i < ($formula_length - 1)) {
-                $this->lookAhead = $this->formula[$i + 1];
+                $this->lookAhead = $formula[$i + 1];
             } else {
                 $this->lookAhead = '';
             }
 
-            if ($this->match($token) != '') {
+            if ($this->match($token) !== '') {
                 $this->currentCharacter = $i + 1;
                 $this->currentToken = $token;
 
@@ -1021,7 +1024,7 @@ private function advance(): void
             }
 
             if ($i < ($formula_length - 2)) {
-                $this->lookAhead = $this->formula[$i + 2];
+                $this->lookAhead = $formula[$i + 2];
             } else { // if we run out of characters lookAhead becomes empty
                 $this->lookAhead = '';
             }
@@ -1198,8 +1201,8 @@ private function match(string $token): string
     public function parse(string $formula): bool
     {
         $this->currentCharacter = 0;
-        $this->formula = (string) $formula;
-        $this->lookAhead = $formula[1] ?? '';
+        $this->formula = $formula;
+        $this->lookAhead = mb_substr($formula, 1, 1, self::UTF8);
         $this->advance();
         $this->parseTree = $this->condition();
 
@@ -1624,7 +1627,9 @@ public function toReversePolish(array $tree = []): string
             }
 
             // add its left subtree and return.
-            return $left_tree . $this->convertFunction($tree['value'], $tree['right']);
+            if ($left_tree !== '' || $tree['right'] !== '') {
+                return $left_tree . $this->convertFunction($tree['value'], $tree['right'] ?: 0);
+            }
         }
         $converted_tree = $this->convert($tree['value']);
 
diff --git a/tests/PhpSpreadsheetTests/Writer/Xls/Issue4331Test.php b/tests/PhpSpreadsheetTests/Writer/Xls/Issue4331Test.php
new file mode 100644
index 0000000000..77841d8290
--- /dev/null
+++ b/tests/PhpSpreadsheetTests/Writer/Xls/Issue4331Test.php
@@ -0,0 +1,56 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpOffice\PhpSpreadsheetTests\Writer\Xls;
+
+use PhpOffice\PhpSpreadsheet\Spreadsheet;
+use PhpOffice\PhpSpreadsheetTests\Functional\AbstractFunctional;
+
+class Issue4331Test extends AbstractFunctional
+{
+    public function testIssue4331(): void
+    {
+        $spreadsheet = new Spreadsheet();
+        $sheet = $spreadsheet->getActiveSheet();
+        $c3 = '=VLOOKUP(B3,$B$10:$C$13,2,FALSE)';
+        $d3 = '=VLOOKUP("intermediate",$B$10:$C$13,2,TRUE)';
+        $c4 = '=VLOOKUP(B3,$B$10:$C$13,2,FALSE())';
+        $d4 = '=VLOOKUP("intermediate",$B$10:$C$13,2,TRUE())';
+        $sheet->fromArray(
+            [
+                ['level', 'result'],
+                ['medium', $c3, $d3],
+                [null, $c4, $d4],
+            ],
+            null,
+            'B2',
+            true
+        );
+        $sheet->fromArray(
+            [
+                ['high', 6],
+                ['low', 2],
+                ['medium', 4],
+                ['none', 0],
+            ],
+            null,
+            'B10',
+            true
+        );
+
+        $reloadedSpreadsheet = $this->writeAndReload($spreadsheet, 'Xls');
+        $spreadsheet->disconnectWorksheets();
+
+        $worksheet = $reloadedSpreadsheet->getActiveSheet();
+        self::assertSame($c3, $worksheet->getCell('C3')->getValue());
+        self::assertSame(4, $worksheet->getCell('C3')->getCalculatedValue());
+        self::assertSame($d3, $worksheet->getCell('D3')->getValue());
+        self::assertSame(6, $worksheet->getCell('D3')->getCalculatedValue());
+        self::assertSame($c4, $worksheet->getCell('C4')->getValue());
+        self::assertSame(4, $worksheet->getCell('C4')->getCalculatedValue());
+        self::assertSame($d4, $worksheet->getCell('D4')->getValue());
+        self::assertSame(6, $worksheet->getCell('D4')->getCalculatedValue());
+        $reloadedSpreadsheet->disconnectWorksheets();
+    }
+}
diff --git a/tests/PhpSpreadsheetTests/Writer/Xls/NonLatinFormulasTest.php b/tests/PhpSpreadsheetTests/Writer/Xls/NonLatinFormulasTest.php
new file mode 100644
index 0000000000..d8ec963181
--- /dev/null
+++ b/tests/PhpSpreadsheetTests/Writer/Xls/NonLatinFormulasTest.php
@@ -0,0 +1,65 @@
+<?php
+
+namespace PhpOffice\PhpSpreadsheetTests\Writer\Xls;
+
+use PhpOffice\PhpSpreadsheet\Cell\DataValidation;
+use PhpOffice\PhpSpreadsheet\Cell\DataValidator;
+use PhpOffice\PhpSpreadsheet\NamedRange;
+use PhpOffice\PhpSpreadsheet\Spreadsheet;
+use PhpOffice\PhpSpreadsheetTests\Functional\AbstractFunctional;
+
+class NonLatinFormulasTest extends AbstractFunctional
+{
+    public function testNonLatin(): void
+    {
+        $spreadsheet = new Spreadsheet();
+        $worksheet = $spreadsheet->getActiveSheet();
+
+        $validation = $worksheet->getCell('B1')->getDataValidation();
+        $validation->setType(DataValidation::TYPE_LIST);
+        $validation->setErrorStyle(DataValidation::STYLE_STOP);
+        $validation->setAllowBlank(false);
+        $validation->setShowInputMessage(true);
+        $validation->setShowErrorMessage(true);
+        $validation->setShowDropDown(true);
+        $validation->setFormula1('"слово, сло"');
+
+        $dataValidator = new DataValidator();
+        $worksheet->getCell('B1')->setValue('слово');
+        self::assertTrue(
+            $dataValidator->isValid($worksheet->getCell('B1'))
+        );
+        $worksheet->getCell('B1')->setValue('слов');
+        self::assertFalse(
+            $dataValidator->isValid($worksheet->getCell('B1'))
+        );
+
+        $worksheet->setTitle('словслов');
+        $worksheet->getCell('A1')->setValue('=словслов!B1');
+        $worksheet->getCell('A2')->setValue("='словслов'!B1");
+        $spreadsheet->addNamedRange(new NamedRange('слсл', $worksheet, '$B$1'));
+        $worksheet->getCell('A3')->setValue('=слсл');
+
+        $robj = $this->writeAndReload($spreadsheet, 'Xls');
+        $spreadsheet->disconnectWorksheets();
+        $sheet0 = $robj->getActiveSheet();
+        self::assertSame('словслов', $sheet0->getTitle());
+        self::assertSame('=словслов!B1', $sheet0->getCell('A1')->getValue());
+        self::assertSame('слов', $sheet0->getCell('A1')->getCalculatedValue());
+        // Quotes around sheet name are stripped off - harmless
+        //self::assertSame("='словслов'!B1", $sheet0->getCell('A2')->getValue());
+        self::assertSame('слов', $sheet0->getCell('A2')->getCalculatedValue());
+        // Formulas with defined names don't work in Xls Writer
+        //self::assertSame('=слсл', $sheet0->getCell('A3')->getValue());
+        // But result should be accurate
+        self::assertSame('слов', $sheet0->getCell('A3')->getCalculatedValue());
+        $names = $robj->getDefinedNames();
+        self::assertCount(1, $names);
+        // name has been uppercased
+        $namedRange = $names['СЛСЛ'] ?? null;
+        self::assertInstanceOf(NamedRange::class, $namedRange);
+        self::assertSame('$B$1', $namedRange->getRange());
+
+        $robj->disconnectWorksheets();
+    }
+}