Unfortunately I would put money on this would never happen. Being in a party sets loot tag, if it’s auto soulbinding and not even giving an option to trade. It sounds like the loot tag isn’t applying.
So no loot tag… no proof of kill.
You are better off doing what you are doing and advocating and bringing attention to these mounts that aren’t allowing group trading.
Rofl yea. I don’t know how I miss these super trivial achievements.