The Chickens that spawn as part of a Chicken Jockey are no longer spawning with the "IsChickenJockey" tag set to 1b. Instead, the Baby Zombies are simply spawning as generic passengers.

The main issue with this is that it is allowing the jockey chickens to lay eggs, resulting in large clusters of Zombies holding eggs being unable to despawn- similar to what happened in 1.7 when the Chicken Jockeys were first added. I believe this to be the cause of MC-231977.

I have attached an image of the data from a naturally spawning Chicken Jockey that I gathered using the /data command. As you can see, "IsChickenJockey" is set to 0b, and a passenger's data can be found.