When mobs that walk (villagers, foxes, drowned, or more related) find a route over water, they assume that water is a full solid block. This leads them to believe that a path through water to an area one block high is valid, leading to cause MCPE-77646.
Video Example
Comments 4
In other words, the problem is that mobs try to path out of water onto blocks that are 1 block up from water level? Then if the block has water under it, they will go under the block and drown.
I can confirm for 1.16. I’ve had multiple mobs did that I needed because of this.