Code change:
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I believe this may be the core cause: The value for hcLight here is just an empty string
Thanks for investigating, looks like this color got forgotten when creating the Light HC theme. I took the same color as used in the overview ruler

Related discussion:
text: export const editorBracketHighlightingForeground5 = registerColor('editorBracketHighlight.foreground5', '#00000000', nls.localize('editorBracketHighlightForeground5', 'Foreground color of brackets (5). Requires enabling bracket pair colorization.')); export const editorBracketHighlightingForeground6 = registerColor('editorBracketHighlight.foreground6', '#00000000', nls.localize('editorBracketHighlightForeground6', 'Foreground color of brackets (6). Requires enabling bracket pair colorization.')); <del> export const editorBracketHighlightingUnexpectedBracketForeground = registerColor('editorBracketHighlight.unexpectedBracket.foreground', { dark: new Color(new RGBA(255, 18, 18, 0.8)), light: new Color(new RGBA(255, 18, 18, 0.8)), hcDark: new Color(new RGBA(255, 50, 50, 1)), hcLight: '' }, nls.localize('editorBracketHighlightUnexpectedBracketForeground', 'Foreground color of unexpected brackets.')); </del> <ins> export const editorBracketHighlightingUnexpectedBracketForeground = registerColor('editorBracketHighlight.unexpectedBracket.foreground', { dark: new Color(new RGBA(255, 18, 18, 0.8)), light: new Color(new RGBA(255, 18, 18, 0.8)), hcDark: 'new Color(new RGBA(255, 50, 50, 1))', hcLight: '#B5200D' }, nls.localize('editorBracketHighlightUnexpectedBracketForeground', 'Foreground color of unexpected brackets.')); </ins> export const editorBracketPairGuideBackground1 = registerColor('editorBracketPairGuide.background1', '#00000000', nls.localize('editorBracketPairGuide.background1', 'Background color of inactive bracket pair guides (1). Requires enabling bracket pair guides.')); export const editorBracketPairGuideBackground2 = registerColor('editorBracketPairGuide.background2', '#00000000', nls.localize('editorBracketPairGuide.background2', 'Background color of inactive bracket pair guides (2). Requires enabling bracket pair guides.'));