Cards missing from PayPal checkout on Delta.com?

I've been using PayPal checkout for years. I have many cards linked. Never have I seen this issue before.

I'm trying to check out on Delta.com using PayPal (first time I've tried this on that site). And for some reason, none of my 4 Amex cards are showing up (my Chase & Capital One cards do show). No error message, just simply not there. They *do* show when I try checking out on other sites, like eBay. I've tried 2 laptops, 1 phone, & 3 browsers. I've contacted Delta & Amex, neither of them have any idea what is going on, nor any clue on what, if anything, they could possibly do to address it.

Normally, I wouldn't care. The issue is that I just got the Amex Delta Business Gold via the offer at the end of the Delta checkout process. I was not provided a virtual card number, but I was able to link it to PayPal. In order to actually get the $300 bonus credit, I need to charge the flight to that card...

I'm curious if anyone has ever seen anything like this? A specific bank's cards not showing as available in PayPal checkout on a specific retailer's site, but fine everywhere else.