The bug

Although wearing leather boots prevents you from sinking into powder snow, prevents you from sinking into it, the leather horse armor doesn’t prevent horses from sinking.

How to reproduce:

1. Place powder snow
2. Get a tamed horse, you can use /summon minecraft:horse ~ ~ ~ {Tame:1b}
3. Put leather horse armor onto it
4. Move the horse into the powder snow
    The horse sinks, despite wearing leather horse armor.