Whether that's a good thing or not is another story. From another forum:

"Anyone else frustrated with the new Uber Cash system that includes the Amex Plat credits, or know of a workaround? Used to be that the $15 credits applied automatically to any charge no matter the price. The "Uber Cash" system requires that your Uber Cash balance be larger than the cost. So if I have a >$15 trip, I either can't use my credit or I have to "top up" my Uber Cash balance."

and

"I was trying to use my $15 uber credit last weekend on a $23 ride. They said I couldn’t use it because I didn’t have enough Uber credit. Is that real? I can’t use the $15 and pay $8 on a credit card?"

Sounds like yet another Uber "customer un-friendly" policy is slowly rolling out.