Netherite Horse Armor does appear on regular horses but is paler than Iron armor which should not be the case, it has an extremely misleading color from what it's supposed to look as an item.
Pictures below.
This is on a world with no behavior or recourse packs, and does not have experiments on, however the bug was also on experimental worlds.
What experiments do you have enabled? You mentioned an experimental world.
I tried replicating the bug with both vibrant visuals and fancy graphics and couldn't get the horse armor to appear pale.