As I started playing the latest snapshot, 21w03a, I realized that my chickens were slowly escaping through the glass barrier surrounding their pen. I utilize entity cramming for both my chicken farm and a cow crusher. I gathered some of my escaped chickens within some fences and noticed that when the baby chicks grew to adults, they got pushed out of the fence wall. I then tested this with the cow crusher, and the same occurred. After the cows grew to adults, they began escaping the one block of space they had been crammed in, and I had about 10 wandering around the yard. I believe this is due to the bug fix MC-9568 that is supposed to prevent mobs from suffocating within walls. I had never noticed that bug before, nor had problems with mobs escaping until today.

I reverted my seed back to 20w51a, and see that it is fine again.