iMessage/Texting Glitches After Upgrade to 18.1.1

Hello! I manage cell phones for my company and have had some folks upgrade to iOS 18.1.1, but we're noticing an error that I haven't been able to resolve. The upgraded phones are in various group chats, and some of them work fine, but other chats have now been separating the users' contributions into separate text messages outside of the group thread, which is annoying. These phones are also no longer are using iMessage, instead just RCS/SMS.

I've tried the following:

• Power-cycling each device
• Toggling iMessage on and off (and turning off and on again between toggles)
• Comparing the Messages settings to a not-updated phone, which are identical except for the update
• Leaving the affected group chats and rejoining
• Deleting a chat thread with one person and then sending a new message

None of these things worked. Due to the nature of our work, we need the encryption and security that iMessage offers, and I'm really unsure how to fix this mess. Does anyone have any idea?