Not certain this is a bug, but it's certainly inconsistent.

There's a 2-part fate in Coerthas Western Highlands: Unsilent Vigil followed by Pico de Gallows.

The first part is waves of archaeornis, the second part is a boss (Picolaton) who spawns with three more archaeornis.

The three archaeornis that spawn with the boss mob in the second part can drop archaeornis skins, however the archaeornis in the Unsilent Vigil stage have no drops.