Yesterday, I booked a domestic first class DL flight through Capital One Travel to use the $300 credit.
Today, I received this email:
We were unable to confirm your seat selection.
You can still select seats directly through the airline’s website. If funds are on hold for this booking, they’ll be released in 7-10 business days. If you’ve already been charged, you’ll get a refund to your original payment method, typically in the next 1-2 billing cycles.*
...which is inaccurate. delta.com shows that I have the seat that I requested.