I believe this is the right way to report this, but if not, please let me know what to do differently. Thanks!
I was just in a game (which might have been #136646053 on oregon, although I don't believe it to be a sporadic bug) with Mirror and Ride, which I bought in that order. I would have expected it to get 2 Horses, but only got one. I confirmed this by looking at the count of horses, which went down by 1.
Mirror says "the next time you gain an Action card," without specifying that the card must be bought.
Therefore, I think I should have gotten two horses, since that was the next time I gained an Action card...
Thanks for the report, TheStorm042, and happy new year!
However, the behaviour you experienced is correct. Unless called out by by name (and exceptions for some special piles like Spitits and Loot), you cannot gain a non-Supply card. The 2nd Edition base rulebook says
(https://i.imgur.com/A2kKji0.png)
Since non-Supply cards are not present for a few initial expansions, it's easy to forget about the "Supply" restriction. Expansions with non-Supply cards usually say that they can't be gained except by effects explicitly naming them. Here is the part from the Menagerie rulebook:
(https://i.imgur.com/dkPDhxw.png)
It uses Menagerie expamples, but what it says about Falconer and Displace applies equally to Mirror.