I forgot to comment on your suggestion to "Clean Build Folder". Yes, sometimes that works, sometimes the abi.json and other IDE-maintained files (invisible to us) get hosed and it's necessary to close and re-open the project. In the worst case, quit and restart Xcode. All quite counterproductive, in an escalating way.

There's really nothing that a video would show -- the change happens sporadically in a far removed part of the code while I'm editing (either in a separate file, or offscreen in the editor I'm working in. From my experience of filing bug reports with Apple, this will get closed immediately, no reason, no request for more info. Just dismissed as operator error.

I don't open files in the history tab, but I frequently "Jump to Definition" since I'm using a new dependency, the API of which I'm not familiar. Perhaps that's what is trigger the bad behavior. I also see that when I jump into underlying code, and then use Command-Control-left arrow, the current editor tab closes, yet the jumped into file takes over the editor tab of MY source file. So counterproductive.

Ha, I overlooked the requirement for macOS 12.0+ Beta. Thanks. Can't upgrade at the moment. I'm accepting the answer, even though I cannot verify it.