I'm in the same boat as most of you. Upgrade to 12 Pro and passed down my 11 to my wife. Few days later she asked why I wasn't responding to the group message thread we have for my family and then I noticed I was not getting them.
Did all the regular troubleshooting: reset network settings, disable mms/group then reenable, deleted old conversation threads, even reset and setup device as new. I was running the iOS 14.2 beta on the 11 prior to update and did not notice the issue. Even now my wife has the 11 running the 14.2 beta and no issues there. T-mobile checked all account settings and it appeared fine, they recommended getting a new 5G SIM card which I did last week but issues remain.
At this point I have to assume the problem is one of two possibilities:
1. there is a bug in iOS 14 that is only related to the new devices (iPhone 12) or
2. the new cellular radios/chips in the iPhone 12 are having issues with all carriers and this needs to be worked out between the carriers and apple