Minecraft 1.16.5 Forge 36.2.35 Dungeonsmod 1.4.43
If bosses are killed by creative player, they will drop less items. For example, crawler will not drop crawler armor if it was killed by creative player.
This may be deliberate, but it is not good for testing.
I suggest that all dropped item should obey the loot tables. And there is a predicate to get whether the killer is creative player and control whether the item should be dropped.