While playing with a friend, who was on a seperate console but in the same room, in 2v2, the match started with our opponents missing from my screen, Super bars and everything. I was visible to my friend, including all moves I was making, but my character could not interect with anything not could the level or opponents interact with me. Once my friend got beaten from the 2 on 1, we were forced to quit the match. It's only happened the one time but it seems like a pretty major bug to me if it repeats.
I was in every single beta and I can tell you, this game changed dramatically for the better.
I had the invisible opponent bug in 3 out of 4 matches yesterday after I had installed my new router, because I had failed to notice that I only had NAT 3. After changing the settings of the router and getting back to NAT 2 everything worked perfectly again.
In two of the matches only one other player had been visible to me, while the remaining two could neither be seen or attacked, nor could they apparently attack or see me (rather game-breaking given the fact that they were all stock matches), forcing me to quit said matches. In the third match only one guy was absent, although the situation was basically the same, although he was visible on the score screen after the match had ended (not a stock match this time). I only really noticed that he was invisible to me rather than missing completely after seeing the other two players getting hit out of nowhere.
If any developer happens to be reading this, I believe NAT 3 is what is causing this in a lot of cases, although I am not sure if this issue can be fixed on the developer's side. Either way, it might be worthwhile to check this issue out and perhaps pass the information around that this is apparently to blame in some (or most) of the cases. Although I did hear NAT 2 players complain about this as well, but perhaps only because NAT 3 players were in their matches.
TL : DR Version:
NAT 3 could possibly cause invisible opponents bug (it did so for me), NAT 2 can fix it again (again, it did so for me).