Group iMessage fails with certain pairings, works only with all 3 contacts

I have 3 contacts (I’ll call them Larry, Curly, and Moe), we all have iPhones and all have iMessages enabled. I can message each of them individually just fine.

If I make a group with Larry and Curly, both their names turn red and they don’t get the messages. If I make a group with Larry and Moe, same thing. But, I can make a group with Curly and Moe just fine.

Here’s where it’s weird. If I make a group with all 3 of them, their names stay blue and everything works fine. If I remove either Curly or Moe, the remaining names turn red. Add the third one back in, and they’re all blue again.

So why does Messages hate me having Larry in a group with just one other person? Why would his name be red with either of the others, but turn blue once all 3 are in the group? And as I mentioned, all four of us have iPhones with iMessages enabled. All with iOS 18.6.2. The only difference I can think of is Larry uses Tello as his carrier, while Curly, Moe, and I use T-Mobile. But why would that make a difference?

[Re-Titled by Moderator]

Original Title: Weird Messages behavior with red contacts