Sometimes, mobs seem to freeze and stand still. They also completely stop interacting with their surroundings. I am unable to hit, shear etc. They also stop moving. If I save the game and exit, then reload the same world, the frozen mob is gone upon re-entering world. On a single occasion, a "frozen" mob just disappeared while I was looking at it.

On a single occasion, I believe I was hit by a zombie which I was unable to see (not "invisible" as the status effect, but "not there" in a way). It zombified a villager, that is how I believe it was a zombie. Also, the zombie seemed to be stuck on the same spot of ground, it would not move to attack me, yet attacked when I got too near to the spot where it must have been standing. It was able to zombiefy the villager because I removed the block of dirt it was apparently standing on, giving it access to the villager (who was stuck in his house).

On another occasion, I found a zombie standing in flowing water, oblivious to its surroundings (iron golem was near and did not attack) and unkillable.

Thinking to the issue MCPE-101202 this might be similar (possibly the same), considering the issue of villagers being "ghosts and statues". Possibly, what I see is a ghost, while the statue is long gone (or attacking me while I cannot see it).

Sometimes, blocks are affected in a similar way: I have had trouble placing dirt blocks as they kept disappearing as soon as I placed them. This went on for some minutes, then I was able to place dirt blocks again. Hard to say if this is the same glitch.

I have not encountered this behaviour prior to 1.16.100.