I tried searching to see if anyone else has this problem, but I couldn't find any combination of terms that yields anything close to what I'm experiencing.
Periodically when composing a text message with the phone rotated (ie: all I can see is the keyboard and text box) my phone will randomly exit the compose view and return to the message view. It will randomly highlight a message farther up in the chain of messages than what I am replying to. If I return to composing the message in the horizontal mode it will either immediately return me to the previous screen or after a random period of time do so. Sometimes it returns to normal on it's own, sometimes I have to power cycle.
Has anyone else experienced this?