Family group texts keep merging and unmerging

We have two family group texts, one with my spouse's family that has 8 members (let's call it Group#1) and one with my family that has 10 members (let's call it Group#2). Four of us are in both chats: my spouse, 2 daughters and me. Group#1 and Group#2 keep merging and unmerging, merging and unmerging, just for the four of us who are in both chats. This is not happening to the people who are only in one or the other group. All in the same minute - and this has happened now about 8 times since September - one of the group texts will have a list of people added to the group and a list of people who have left the group. For the 4 of us then, ALL the messages from both chains will be in one text chain, labeled either Group#1 or Group#2. We have tried all the advice that we could find on the internet and none of it has worked. How do we stop this group merging?