I've been trying to get a drowned chicken jockey to spawn, with little success. I'm aware that a naturally spawned chicken jockey will separate into a chicken and baby drowned, see also MC-209784. As far as I know from the MC gamepedia wiki, this is expected behavior.

So I have been using a second mechanic to get a baby drowned jockey, by having the spawned baby drowned check for a nearby unoccupied chicken. This mechanic has worked for baby zombies and baby zombified piglin. Sadly, as seen in the attached picture, the drowned jockey spawn for this second mechanic seems bugged. There you can see that the baby drowned is standing inside the chicken, rather than mounting it.

Note: for command block-spawned drowned, the naturally spawned drowned jockeys are persistent: they don't separate into a chicken and drowned. Also, command block-spawned baby drowned will easily hop onto a chicken.

Steps to Reproduce:

1. Make a water column above a river biome, allowing drowned to spawn. Here I used flowing water.
2. Have a nearby chicken (or several) placed on glass blocks, making sure the glass doesn't touch the water.
3. In 0.25% of the spawns, a baby drowned will spawn that should attempt to mount a chicken.

Observed Results:

The baby drowned spawns at the location of the chicken, but does not mount it.

Expected Results:

The baby drowned should mount the chicken and become a chicken jockey.