Messages splitting group thread and behaving oddly with Pins

In the last couple of weeks, a previously existing Messages group text thread has split into 2 groups. Details:

1) Group of 8, seven with iPhones and one Android, but it has worked normally for some time (including the usual limitations of one “green bubble” user).

2) Group name and membership has not changed since a few weeks before the split occurred.

3) The split does not occur at the same point among users or devices. Even my own iPhone and MacBook (both purchased late 2024, MacBook Air Sequoia 15.3.2, iPhone 16 Plus iOS 18.3.1, Verizon.) split the thread at different points, and replies to either thread by other users appear to be a coin flip as to where they appear. Even Tapbacks do not always go to the thread that contains the original message or pic that was tapped.

4) Could this be an iOS/macOS issue related to Pins, not to cell carriers, MMS/RCS, etc.? Here is why I ask. I have RCS toggled off in iMessages. Yet when I Pin EITHER of the split threads in Messages, it says the pinned thread is now an RCS thread, but the unpinned one is not. But when I then unpin EITHER thread, it switches back to MMS. This is repeatable; I did it several times to confirm. When I try to pin BOTH threads, Messages refuses to let me, and sends the second one I tried to pin back to the regular text list. Is this possibly due to other users’ RCS settings, or is this a glitch with the latest update to Pins/Messages?