This has happened twice when I used one of Valentine's Countervenom BBs. Each time it's when the opponent gets up after a knock down and then uses a special move right when I use the BB.
The glitch turns them into missingno. like back in soft launch. The only difference is that now they're fully invincible and trying to attack them just pushes them across the stage. As a result the match becomes impossible to win, I either have to wait until the time runs out or restart the game. Also if I try to tag in another teammate, the teammate won't appear at all and tag ins will still be set on cooldown.