Immobile mobile - 'skeleton'

Expectation: visible Skeleton should move, react, shoot, etc.

Reality: Some visible skeletons [random] did not move/react/shoot or respond to user interaction other than 'blocking' action through their block.

Random skeleton (see screenshot) appears to be a ghost of a mob - other mobs in the area are active (I had just killed a zombie off screen). I was able to dig around the skeleton (but not through him), hence why he is 'floating' in a pit.

Restarting the server made the skeleton disappear (but I think that resets all mobs anyway).