When a paragraph of text does not contain any strongly directional characters, then neutral characters will adopt the direction of the paragraph itself. This is not defined by the UBA and typically based on some sort of heuristic. In QTextEdit, the paragraph direction appears to be based on the direction of the current language (if detected via translations) or it defaults to LTR.

In Qt Quick's TextEdit the direction is based on the direction of the first paragraph of text.

The difference here is a bit unfortunate, but it is likely too late to change any defaults. However, we should give the user more control over how this is determined in Qt Quick. At the very least, overriding the default layout direction of the editor should be possible (like you can with QTextOption in Qt Widgets) but maybe also a way to switch between different types of heuristics would make sense?