Horses still drop horse armor when doMobLoot, doEntityDrops and [vanilla] datapack are disabled

Make sure all entity drops, mob drops and the  [vanilla] datapack are disabled.

/gamerule doMobDrops false && /gamerule doEntityDrops false && /datapack disable vanilla

spawn a horse:

/summon minecraft:horse ~ ~ ~ {Rotation:[90f,0f],Silent:1b,NoAI:1b,Invulnerable:1b,ArmorItem:{id:"minecraft:golden_horse_armor",Count:1b}}

Kill it.

It drops its armor.