I just realized after looking at the IDE in D11 dev system, that the D12.1 IDE is not rendering the form using CSS settings when Web Designer=True! I suppose that means that the WebView2 installation on the Win11 PC is faulty even though there is no error message anymore when I enable Web Designer. Hmm.
EDIT: Choosing to repair WebView2 in the Installed apps menu, restored the correct Designer view in the IDE, but didn't affect the stray breakpoint action.