On a server in the 21w11a snapshot, I was attempting to breed two trader llamas from a wandering trader, but I noticed behavior that I believed was unintentional.

Upon further testing in a singleplayer world, I attempted the following in 21w11a:

1. Spawn an animal that can be tamed via riding (tried with llama, trader llama, horse, donkey, and mule)
2. Wait 30 seconds while riding
3.  The animal does not eject the player, nor does it become tamed, and no particles appear. The entity data shows that the animal is not tamed.

I then used the same process in a vanilla 1.16.5 world, with the following result:

 Within 30 seconds, the mob either ejects the player or becomes tamed, with small smoke particles or heart particles, respectively. The mob can become tamed as seen in its entity data.

Taming seems to be the only mechanic affected. The untamed mobs cannot be equipped with saddles, carpets, or chests, and they cannot be bred. Taming the animals using the /data command allows them to be equipped with saddles, carpets, and chests, and they are able to breed.

This bug affects llamas, trader llamas, horses, donkeys, and mules, but does not appear to affect skeleton horses.

Mobs that do not need to be tamed to be ridden or equipped, such as pigs and striders, are not affected by this bug.

(The screenshots "2021-03-31_10.42.37.png" and "2021-03-31_10.42.51.png" are taken in 1.16.5, whereas the rest are in 21w11a.)