Two different Message threads for same Group

I have a Group with two other friends. I sometimes send my Messages to this group from my iPhone (an XR on iOS 14.2) and sometimes send or receive Messages to this group on my MacBook Pro (on Catalina 10.15.17). For some reason, I have two different "threads" for this Group. Sometimes the Message is in one of the threads and sometimes it is on the other "thread". Why aren't they "combined" as only 1 thread? How do I fix it? What causes the two separate threads? Is there something wrong with my Message settings? Or something wrong with my Contacts? My Contacts are synced in iCloud.