There seems to be some strange bug with the muck flies.. they will attack as normal for a time but, eventually, they seem to almost "lose sync". The reason I say this is they just stop attacking. You can continue attacking them but your own attacks will begin missing 100% of the time (I assume some kind of anti-cheating code?)

If you move, however, they will swarm around for a second and adjust position and resume attacking. The result is that you have to constantly move around to keep them behaving normally and combat-able as they should be!