The bug

Many mobs that are being ridden by other mobs (e.g. Spider Jockeys, Skeletal Horsemen, Creepers or zombies riding other zombies/creepers/skeletons) move extremely fast. Creepers that are being ridden also continue to move instead of staying in place when exploding. This applies for most ground-based mobs except for mobs ridden by slimes and magma cubes, which are affected by MC-93276 instead.

To reproduce

Summon any mob riding another mob (e.g. with the following command):

summon creeper ~ ~ ~ {Passengers:[{id:silverfish}]}