I've been doing some more testing (Kind of had to considering what I do on FSE requires me to use BB code) and I've also been thinking about the problems. I think the problem lies in an error with the BB code. Maybe the new update changed the way or priority of how BB code was read? Like (Center - Times new Roman - size 12 - B) in that order before the update worked, but now the update changed it so it doesn't work unless it's in, say, reverse order? Drafts worked for me when I didn't use any BB code (besides spoilers), and using BB code also kills regular posts and PMs, so it's more than likely a BB code problem... or something. I don't really know
Perhaps rolling back to before whatever was updated would fix?
Major Edit: PM-ing is also broken in a major way. I was PMing myself a message for the Forum Game I'm running. I had the tab open and was (and still am) constantly editing it. I had to PM DoctorWarband something that I needed him to do, so I opened a separate tab and PM'd him. I typed the message and hit preview to make sure it worked. The message worked but the text in the PM I was sending to DoctorWarband changed to the text of the other Tab I had open.
I did not tamper with it in any way, I merely hit preview. What is being previewed is my PM to DoctorWarband, ie what I typed in that PM. What appears in the actual typing box is the text for my other PM. I have permission from DoctorWarband to post the PM to him, I can post screenshot as well if needed or you can ask him.
Why is FSE breaking?
My PM to DoctorWarband after hitting preview