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.

Time: n/a
Frequency: Regular/every fate spawn
World name: Phoenix
Character name: Taishelo Firialocin
Retainer name: n/a
NPC name: n/a
Monster name: Archaeornis
Class/Level: Drg/60
Party or solo: Party (Solo with chocobo)
In-game time: N/a
Area and coordinates: Coerthas Western Highlands ( 19.2, 15.3)
Housing: N/a
Steps:
1. Wait for Unsilent Vigil FATE to spawn.
2. Note that archaeornis have no drops
3. Complete Unsilent Vigil Fate
4. Kill archaeornis that spawn with Pico de Gallows FATE
5. Note that there are drops