i had an villager farm in an snowy environment with snow on the ground on the night i saw that as soon as a zombie got track of an villager and started walking on the snow with some height difference the zombie started jumping and did not move very well, and then i noticed some creepers do the same.
to replicate this bug you can spawn some villagers on an hill in some snowy environment where theres snow on the ground, then spawn the zombies on the ground so that the zombies will try to get the villagers but have to jump on snow covered blocks and you will get the same result like the screenshot i demonstrated with