I have two controllers that both work fine and have full battery.
I started playing online and played like 15 games in a row fine, then my controller started disconnecting as soon at the beginning and the middle of fights. It got to the point where it has disconnected in 100% of my last 8 or so fights within the last 48 hours.
If I play single player or local multiplayer, I do not have this issue.
Also, strangely, much of the time I can still navigate the menus, but cannot move my fighter after the controller reconnects.