I've done a purge, reboot, reinstall (1.41) & reboot. I'm still having issues.
I'm not including Slack in my group now. Here's my Application Grouping info:

It's still not working 100% as I keep getting this happening:

Edge is "popping out" into its own group, and when I minimise Edge then maximize it, it's back to original grouping again. I also see the fuzzy patch in the top-right of the screen.
The previous problems I've reported don't seem to happen if I don't group Slack. However, if I move Slack to the same monitor as my group, it adds on to the group automatically, even if I don't maximize it. Am I doing something wrong here?