Since all these mobs are zombies, meaning they can all have jockey variants. BUT, when the jockey tries to spawn, the zombie on top of the chicken is an adult.
how to replicate:
make a world, go into the server config folder, open the forge config and turn up the baby zombie rate to 100% (this will take effect instantly, no need for a reset).

Even the bosses have a jockey variant.

those zombies do not have a baby variant, the only ones that do have a baby variant are burned, frostbitten, swampy and zaps

that's the thing, even without baby variant, they still spawn with chicken jockeys, while they shouldn't because they don't have baby variants.

hmmm very well, i shall then remove that chance