Loot restrictions for baby mobs remove not only their default loot table but also any items they have equipped (through commands). The armor is there, and if you wait for them to grow up, it will be dropped.

This happens to any mob that becomes a baby through a negative Age tag rather than "IsBaby:1", for example villagers, piglins, hoglins, pigs. It worked for any baby mob I tried, to any piece of equipment I tried, on any slot I tried: armor, main hand, off-hand.

Example of reproduction:

1. This command summons a baby hoglin about to become an adult in 5 seconds. It has equipped a diamond with a 100% drop chance:

/summon hoglin ~ ~ ~ {Age:-100,ArmorItems:[

,{},{},{}],ArmorDropChances:[2F,0f,0f,0f]}

2. If you kill the baby hoglin, it won't drop the diamond.

3. If you wait for 5sec and then kill the adult hoglin, it will drop the diamond.