I have this issue as well, and it’s becoming infuriating. A few group threads that I’m on have become a garbled, nonsensical mess on iPhone 12 Pro AND my iPad Pro.
Things I have tried, all to no avail:
-Update to 14.2 (18B92) on iPhone and iPad
-Restart of both devices
-Made sure MMS is turned on (also turned it OFF for a day and back ON)
-Tried changing “you can receive messages to and reply from” to include my email address, and mobile number, back and forth
-Turned off text forwarding to my iPad, and turned back on
-Send as SMS turned off, eventually turned back on
-Group messaging turned off, then on after