Not an answer, but just noting there is another similar thread:
I have had some muck-ups when I had sent group “several individual SMSs” and later group “single MMS” to the same people. When they replied then I get it to different threads, they don’t know when they send to one or the other, etc. It really was a mess, not certain we ever got it completely sorted, we all moved to Signal in that one case. Others that I only reply to group MMS are fine on both sides so it seems.
Not sure if there is a bug or just a limitation of iPhone iMessage ↔ Fossify Messages or other.