Sometimes, probably when you relog or sleep, mobs become invisible. Their only get damege if you hit them from a bow or if you hit a visible mob with a sword and they stand near to him. It's happends sometimes me and me friend, I'm sure it isn't fault server because it happend to me also on singleplayer.

There is a link to video - https://www.youtube.com/watch?v=E4yjbqBly2g