Can't use Amex Uber credit on past orders anymore?

I can switch the 10.26 ride to regular topped-up Uber Cash and gift cards just fine. Is the rep just making things up, or is the Amex credit special? I tried two different reps and neither could do it.

Also, the ride is $25 — could it be that the credit doesn't cover the full ride amount, so it can't be applied? The ones that worked before were all fully covered.