Our imessages were blue, now green, but fixable by deleting the tread - why?

ParsleyNThyme wrote:

I text regularly with my husband, we both have iPhone 12s. Our tread is normally blue and grey. Occasionally our responses will turn green without us changing anything. We have waited for it to change back, rebooted, checked connectivity - none of these things seemed to be the reason. What does work is deleting our thread, only one of us has to do this, and we are both back to blue...

Why is this happening? How can we get it to stop happening?

When imessage is unavailable it will send it as a SMS, could be due to either one of you being in an area with a weak wifi or cellular data connection, or have imessage turned off.