I'd be surprised if this hasn't been reported yet, so if it has and I overlooked it then I apologize in advance.

Mobs that are swimming in water will get stuck when they swim into a block that is also occupied by an underwater plant (see attached picture).  The three mobs circled in the picture are just spinning in random directions, and they're not swimming anywhere while they're in a water block that also has plant life in it.

If I break the plant underneath the stuck mob, then the mob will continue to swim to the destination where it was heading before it got stuck.

Is the mob getting stuck in a 'decision loop', where it's deciding a different destination per tick, while it's in a water block that also contains an underwater plant?