iMessage doesn’t work at all for group with SMS + iMessage users, iOS 18.6.2

SMS texts work fine but not if a group includes both SMS and iMessage users. Adding an iMessage user to an SMS causes all addresses to turn red & messages to be undeliverable to any in the group, on any of my Apple devices. I have SMS enabled and message sharing enabled on my iPhone. This was solved over a year ago, but over the last year and with upgrading devices the problem has returned.

Edited to add: The user with which I have this problem does not have an RCS-enabled phone.