A non-wolf armor item with an equippable tag that has a wolf as the allowed entity slot still cannot be given to wolves, but it works fine for horses and llamas.
Steps to Reproduce:
Download and install the provided resource pack.
Give yourself the following item:
/give @p minecraft:diamond[minecraft:equippable={slot: "body",model:"minecraft:royal_pet_armor",allowed_entities:["minecraft:wolf","minecraft:horse","minecraft:llama","minecraft:trader_llama"]}]Right click on a horse, llama, and wolf with the armor in your main hand.
Observe that the wolf does not accept the armor.
Expected Behavior:
The armor should be applied to the wolf's armor slot.
Observed Behavior:
The armor is not applied, and instead toggles the wolf's sit state as if the armor is not valid.
Can confirm.