The bug

Mobs can replace weapons held in their main hand with armor. This does not occur at some point prior to 1.19.3, and might be a side effect of MC-259259.

To reproduce

1. Summon a skeleton.

2. /data modify entity @e[type=minecraft:skeleton,distance=..10,limit=1] CanPickUpLoot set value 1b

3. Give it a diamond chestplate item.
4. The skeleton will correctly place the chestplate on its chest.
5. Give it another diamond chestplate item.

Observed result

The skeleton swaps the bow for the diamond chestplate.

Expected result

The skeleton would ignore the chestplate, as it occurred in previous versions.