The "Lava Chicken" record does not drop when you kill a baby zombies

When I was playing my survival world in version 1.21.7 Release Candidate 2, I noticed that when I tried to kill a small zombie on a chicken, I didn't get a music disc lava chicken. I thought it was just bad luck, but when I tried again, I still didn't get a music disc.

Then I decided to check this bug in another world in creative mode, where I realized an interesting thing:
We can only get the music CD when we kill the little zombie before his chicken, or when we kill his chicken and kill him when he's animation sitting.

How to reproduce:

1) Summon a small zombie on a chicken using this command:

/summon chicken ~ ~ ~ {Passengers:[{id:zombie,IsBaby:1}]}

2) First, kill the little zombie chicken, then wait for the little zombie to get back on its feet and kill it. (You won't get a music disc).
3) Now create a second little zombie and kill its chicken first, but then kill it immediately while it's sits in the air. ( You get a music disc “Lava Chicken”).

I've attached a video for a more detailed overview.

In fact, this bug confuses players, as it is possible for a player to kill a chicken first and then only a small zombie, leading to various disagreements in regular gameplay.