Add Funds to Apple Cash with American Express DEBIT Card in Apple Pay

Hi. I am looking for a definitive answer on whether or not American Express DEBIT cards are compatible with Apple Cash. I am able to add my American Express debit card to Apple Pay and use it just fine. The issue comes in when I try to transfer funds to Apple Cash. The AMEX is my ONLY Debit card, but it seems to not be compatible. I have escalated this with Apple Support many times, but with no success. They are just as confused as me. Would love any insight on whether or not this is something that just isn't supported at the moment.

Thanks!