By some good guesses from me after hours of eventually discovering that ExtraHardMode isn't at fault, it happens to be MobArena at fault here. My guess is this is the offending code:

https://github.com/garbagemule/MobArena/blob/91d426a74b62c8279fcd093b75ca442e98cd1092/src/com/garbagemule/MobArena/ArenaListener.java#L426