Are the 'Rider' medal quests broken?

I ran into the same problem just now. 9/10 for Easy Rider, bought the Aria mount, it stayed at 9/10.

What's weird though, is that for the other two Mount medals, they went up to 12/xx.

Ugh, yet another GMS exclusive medal that's glitched alongside with Creature of the Night/Scared of the Dark...

Try using the plane "mounts" from the airplane guy. They count technically, and if you are lucky, you could make them update your count.