J$1...did you have Tmobile replace the phone or Apple?
I, like all of you, have had the same issue for both my wife and I (both upgrading from iPhone 8 to the 12 Pro). Initially thought it was a Tmobile thing, but this thread has enlightened me to it being across all carriers. Have tried all the same trouble shooting with resetting network settings, toggling MMS/group messages, etc. They seem to work as a (very) temporary fix.
For giggles, I have put the new SIM card in my old iPhone 8 today to see if the problem persists....SO FAR, it has NOT been a problem in the iPhone 8. Only been a few hours, but if it continues to not be a problem on the 8, I may return my 12 Pro until there is a fix :(