If two players are connected p2p, is there a way to detect if one player maliciously severs the connection?
I would assume there can be other peers connected in the session and that could help with detection
But I assume there’s a way for those other peers to communicate differently with the fighting peers, and the fighting peers are able to send the fighting data just between each other
I used to work in the video game industry as a game engine developer. The way we dealt with these situations was in the game logic, not the networking layer. Network connections can drop for any number of reasons and you can’t force a connection to remain open/up even if you want to.
The typical approach in video games is to penalize the disconnecting player by keeping their character in the game frozen for some amount of time after disconnection so that the other players can attack them or do other negative things to them. You could also simply record the disconnection as an automatic loss in the fighting game and have their rank adjusted accordingly.
Cheers! 
Dave