Ok seriously. This thing is rampant.
I'm getting it on a vast majority--a VAST MAJORITY--of games already in-session.
I load in, have absolutely no UI, can't even turn the camera or auto attack.
What the heck is going on?
Issue doesn't seem to present itself when starting new games, only when joining existing ones.
That's fairly common to me as well. You can either:
1. move around and wait a few seconds to see if the UI shows up later (sometimes this works, sometimes)
2. quit the match and try join in again
Since that only happens when you just joined a session, it shouldn't be much of a problem as you won't lose any thing since you haven't gained anything yet 
You won't have access to the menu, so you will need to either use Origin overlay or force shut the game/console