Quick key filling of mask not working

Might be an OS thing.  Sonoma apparently has some issues with Photoshop, but I am a Windows user and not flash with Mac issues

Thanks for the ideas.....already tried them prior to posting. Don't see anything waiting to be tabbed out of, or waiting for input (which would make sense.) Seems like a bug to me. I've been using Photoshop for 25+ years, and never had this problem. It doesn't do it in earlier versions. Going to full screen also seems to clear it btw.

The ding sound would have me looking for a dialog box waiting for input.  You could try hitting the Escape key to cancel the dialog box if there is one.  Pop-ups can be hidden behind other panels, and Alt tabbing (Windows) or minimising on-top windows can reveal a hidden panel if there is one.

I did think about the forground/background focus thing, but I tried to make that hang up Photoshop and failed.  Just to be sure, I expect you already know that the one with the white outline has focus, and you toggle that with the double arrow icon.

Reading your post again, your workaround of coming out and back innto Photoshop has me wondering if something else entirely has focus.  Try opening Photoshop with the Shift key held down to force it to not open third party plugins.  I just keep thinking that something is waiting for input, but I can't think what that might be.