Horses summoned as passengers always spawn as Variant 0

Summary:
Summoning a horse as a passenger of another entity and not specifying a Variant value will always cause the horse to spawn as variant 0.

How to reproduce:

1. /summon <entity> ~ ~ ~ {Passengers:[{id:"minecraft:horse"}]}

→ The horse will always spawn as Variant: 0. ❌

Expected behavior:
Horses variant is random just like when you /summon them normally.