[iMore.com]

As reported by some users through the MacRumors forum, the Magic Keyboard is not charging the iPad Pro when third-party accessories are used.

Full story from the iMore Blog...

[Modelflier]

I had to use an Anker Powerport PD charger which has a separate usbC PD charge socket for specifically charging MacBooks to get my Magic Keyboard to charge, and I also had to use a good quality usbC to usbC charge lead.
Using a normal usbA output from the charger I get the “not charging” message.