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.
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Ah, it was a duplicate after all; my bad, I searched specifically for villagers and kelp so I didn't find MC-136318.
Asteraoth
That's okay. At least you searched. 🙂
I've made this a bit more general as I've just tried it in creative and found that pigs behave the same way, so it's probably all mobs.
Set up was a 4 x 5 tank containing water 4 blocks deep and with kelp filling every water block, with two narrow platforms on either side of the tank and walls around it to keep the mobs in. I watched a villager enter the tank, cross a couple of the kelp blocks, but then get stuck on one and exhibit the spinning behaviour I described. I then lured a pig in with wheat and it did the same thing; neither mob were constantly spinning, sometimes they stopped and looked at me, but they never budged off of the block they got stuck on at all, and even trying to lure the pig off of it with wheat had no effect.
EDIT: Of course, since pigs haven't been lured by wheat for about 4-5 years, that could have something to do with it :facepalm: