On version 1.11, my minions have a disconcerting tendency to turn invisible overnight. It's not always all of them, but even with no minion in sight, I can still hear their response when I call them to follow me or assign a task. I initially realized this when I couldn't place a few blocks (because an invisible minion was in the way). Exiting and restarting seems to fix the problem. (ETA: Well, sometimes.)

ETA: Forge 2189.

This bug has been around for several versions now and i don't know what causes it.