Thank you for the discussion and feedback that you’ve all provided on this. We can confirm that this isn’t a bug.
When crafting a Gem Dragon egg, each Dragon has the same chance to be created but if you have already crafted one of them, this does not prevent you from being able to craft a second.
This feedback has been passed onto the team and we do have plans for these Gem Dragons in the future! But this is working as intended currently.
OminousGMan - Support Human