As I suspected it was the cable - the Apple USB-C to Lightning will fast charge the iPad Pro with my third party PD charger. Started at 38% charge, ~32-33W to 55-60% and it was still taking ~27W at 67% when I unplugged it. The battery life is so good that these tests are annoying
The $25 Apple cable is flimsy junk though, I don't see it lasting more than a few months of use. If they swap them out when they break then that's fine I guess.