/replaceitem does not work with horse armor/llama carpets correctly

The bug

The replaceitem command cannot use the slot.armor.body slot to put horse armor onto a horse, or carpet onto a llama.

How to reproduce

1. Spawn a horse

2. Ride on it until it’s tamed

3. Run /replaceitem entity @e[type=horse,c=1] slot.armor.body 0 diamond_horse_armor

Expected result

The horse should get armor

Observed result

The command claims it succeeded, but the horse does not have armor

Notes

Unlike with MCPE-224269, the slot.armor slot can be used as a workaround. This is the incorrect slot name, though (slot.armor is not a real slot).