I’m not sure if this is a bug or if this intended. Regardless, I think that allowing (some) UI elements to update while the HTML editor is open would be useful. For instance, the preview. This would be particularly useful for things like GitHub - pointtonull/anki-external_editor: Simple substitution of built in html editor with external tool.
I would potentially be willing to implement this if someone would give me some pointers on how this could be done. Also, if I should make an issue on the repo instead, let me know.