can confirm in 1.21 pre-2
can confirm in 1.20.6
this happens on water too: https://youtu.be/X9bwGzOfk-Q
note: this behaviour is wildly different from other mobs, as e.g. pigs can intersect each other's hitbox so there would be practically no issue doing this with large amounts of pigs or other mobs, but due to boats having hard hitbox collision they are much harder to work with in this case
please fix can confirm in 1.21 pre-2