I recently finished two dungeons that offered one of the ones I battled as a companion (for purchase) but unlike in the beta test, they dont stick around. For expample I was just offered Fin Diesel and when I went to look for it in the crown shop it wasnt there. What's up with this? Is this a bug can I or cant I buy?

I might add at the time I didnt have the crowns but I went and got some and the offer was still recended.
What's this about?