Messages turn from blue to green
Yes i know that green is SMS and blue is iMessage, so it's not that....
I find that randomly, often in the middle of a text thread, the messages will go from blue to green and in most cases don't deliver at all. In every case restarting the phone temporarily fixes the problem. Given that a restart always works, it seems not to be about whether one or both phones are logged in. It seems like a software issue.
iphone 13 pro, running ios 15.5 but the problem predates all versions of 15.