This has been happening to me since 18.0, and now on 18.3 and it’s still going. Several weeks ago (while on 18.1), I opened a case with Apple and worked with a support tech who was wonderful, but did not have a solution. At that time she advised that it had not been raised to Engineering yet, but maybe it would work better in 18.2.
I don’t see any activity from Apple on this. I am happy to test solutions. Like everyone else, I tried deleting and rejoining, and several other similar sequences with no good result. I even had one group all delete the group the I recreated it. That didn’t work either.
Is anyone aware of any work by Apple on this specific issue? I have not seen anything.