Loot table condition "killed_by_player" does not recognize when an entity is killed by a player. Any entries using this condition will never drop if killed by a player, however will function properly with the 'loot' command.

This can be observed with vanilla entity loot tables that use this condition, e.g. blaze dropping blaze rods; blaze will not drop blaze rods when killed by a player. Modifying the loot table to remove the 'killed_by_player' condition with a datapack, blaze will drop blaze rods as expected.