It is hard to know but here some points:

I would start here. In this line: #1 RichText.createRenderObject we have this method

@override
RenderParagraph createRenderObject(BuildContext context) {
  assert(textDirection != null || debugCheckHasDirectionality(context));
  return RenderParagraph(text,
    textAlign: textAlign,
    textDirection: textDirection ?? Directionality.of(context),
    softWrap: softWrap,
    overflow: overflow,
    textScaler: textScaler,
    maxLines: maxLines,
    strutStyle: strutStyle,
    textWidthBasis: textWidthBasis,
    textHeightBehavior: textHeightBehavior,
    locale: locale ?? Localizations.maybeLocaleOf(context),
    registrar: selectionRegistrar,
    selectionColor: selectionColor,
  );
}

And here #0 Directionality.of:

static TextDirection of(BuildContext context) {
  assert(debugCheckHasDirectionality(context));
  final Directionality widget = context.dependOnInheritedWidgetOfExactType<Directionality>()!;
  return widget.textDirection;
}

And here debugCheckHasDirectionality: https://api.flutter.dev/flutter/widgets/debugCheckHasDirectionality.html

Do you have access of this computer, try to trace in debugCheckHasDirectionality.