You basically just got unlucky. As you guessed, it sounds like it tried to pull a Mage from another race, which failed as you didn’t have available population for that race. If the Bard had selected an Elf, it would have been deployed just fine.
I think this behavior is correct. The summoning powers are supposed to be a bit of a gamble, to prevent abusive combinations. You wouldn’t want someone to be able, for instance, to stack their deck with nothing but Woodland Bards and Little Wizards (Goblin), and rampage through the entire game unchecked without having to even claim a Goblin settlement.
Similarly, you wouldn’t want the power to only pull mages that you do have the population for. Consider a standard Elf deck (lots of Mages by default), with Bards and about 20 Little Wizards. If the Bard only selected cards you could successfully deploy, this deck would be amazingly powerful in the following way:
- Play a bunch of Bards.
- Claim one Goblin settlement.
- Relinquish all your Elf settlements.
- Use your Bards to automatically deploy a horde of Little Wizards.
- (Optional) Reclaim your Elf settlements.
When the Little Wizards bounce each other back to your deck, you simply redeploy them with the Bards. Meanwhile you’re putting out thousands and thousands of ranged damage every few hours, and spending no gold at all to do it. If you’ve got a couple Eldermages traveling with the group, you’ve also got unlimited Mana.