It also happens to any horse-related mobs, it makes them use the old models
Applying packs in a world (or in global) that doesn't have min_engine_version set in 1.8.0 or higher will revert horses to use the old textures and models, but not just the horses, it also reverts zombified piglins back to zombie pigmen. I think it's just the same issue as MCPE-65496
But the biome textures aren't .tga files, they're .png
(I don't know how to mention a user)
This bug is still in 1.20.10.21. I attached an example addon for testing.
[media]Tapping on a block while holding Bucket of Sand should transform it to an empty bucket. (Btw, bucket of sand isn't meant to place a sand block)
But instead of transforming, it results to:
[Item][error]-Could not transform item into minecraft:bucket, it is invalid.