Not sure if this has always been like this or is recent, or if it is even a bug (seems like it might be, though).
The locked Team-Ups for a match included not just the expected 6 characters (my team and the enemy team), but it also locked Iron Man team-ups, presumably because the enemy Team-Up was of Iron Man. This happened on a Deadpool Daily Quest node, where I only noticed it due to trying to delete a Ballistic Salvo I accidentally accepted.
Screenshots below: