Haven't made a thread in a while but this happened to me too before. I'm not sure if this is wanted or not but it's really annoying, especially since it still shows the IC as posted last even when you change it to OOC in the OP.
I found this bug after posting in character on a forum game. I later went to make a thread in a different forum, and it posted in character (same character as in the game) even when I made sure I had the OOC option selected.
I had to post in another thread out of character so it would post OOC on the thread.