[Proposing as a blocker.]
In testing T208771: Ensure ConfirmEdit applies when the context is an MCR page, I found two not-great outcomes.
If your edit is hard-blocked, it doesn't save (good), you get a response shown as an error (good), but it escapes the HTML rather than rendering it:
If your edit is CAPTCHA-blocked, I imagine it does the same thing, so it won't show the CAPTCHA or its input box and won't let you submit that (though I couldn't trigger that behaviour).