I got nearly the same result when using /summon Spider ~ ~1 ~ {Passengers:[{id:"Boat"}]}. My guess is that the boat is trying to push the mob into the boat, as boats now do, so that they are both riding each other.
The glitched mob
I got nearly the same result when using /summon Spider ~ ~1 ~ {Passengers:[{id:"Boat"}]}. My guess is that the boat is trying to push the mob into the boat, as boats now do, so that they are both riding each other.