Fight Mode stops working(nothing happens when pressing LMB) after rejoining the server, killing player fix this problem but only until the next rejoin.
Mohist 1.16.5(or Magma 1.16.5)
Epic Fight 1.16.5-5.1.1

Normally, It was turned out that the server with a different API (Mohist in your case) has a lot of problems. I recommend using the pure Forge server.