If a villager crosses a body of water and passes through a block where kelp has grown up to the surface of the water, the villager will stop moving and stay on that block forever, rapidly spinning on the spot.

The villager can be dislodged by floating up to it and prodding it free, but it will never leave the block of its own accord unless the kelp is broken.

Just tried this in creative and pigs exhibit the same behaviour. They don't get stuck on every kelp block, however; I've seen them cross a couple and then get stuck on the third, for example, but once stuck only manual intervention can shake them loose.

I've seen this happen multiple times in my 1.14 world as I have a pond with a lot of kelp in it between my main house and the village I built, and one of my villagers has decided to live with me but occasionally makes his way across the lake to visit his neighbours. It's quite amusing to watch him spin forever.