I have Frontier selected with my Aspire card.
I purchased 2 one way tickets 11/29/19 for $44 each. It did not automatically trigger the credit. I chatted with Amex because my credit meter said $25 remaining. I just chatted that I was new to Aspire this year and that I didn't understand how the credits/remaining balance works. The rep said he saw that I had selected Frontier this year and that I had made a $44 purchase. The supervisor credited my account (rep did this without me asking). I need to purchase 2 more tickets. I don't think I'll be so lucky twice, but now I might as well try since Frontier is selected and I have used part of the credit.
I do have a voucher I could use and I have fee-free booking over the phone. I'm tempted to try that route.