if the above suggestions didn't help, submit error report to Samsung Members after reproducing the issue once more.
Who is your carrier? have you contacted them to see if they are able to help?
Just updated my Galaxy Tab S7+. Now any group text messages come through either individually or just directed at 2 people. On my phone S20+ 5G, it works correctly sending and receiving. Prior to the update, the default Samsung messaging app worked correctly. Have restarted both devices several times. Cleared out data and cache. Still doing it. Suggestions?