iPad Pro 11-inch has suddenly stopped charging with the official Apple adapter it came with

Given that your iPad will charge at a slower rate with a standard USB Power Adapter - but not with the supplied 20W Apple Power Adapter that has been demonstrated to function with other devices - this would suggest the the iPad’s charge-controller is likely to be faulty.

When connecting to a compatible USB PD (Power Delivery) Power Adapter, the iPad’s charge controller circuitry negotiates its power requirements with the Power Adapter. If this “negotiation” fails, the Power Adapter will either not provide power, or default to a lower output. Conversely, when using a standard 5V USB Power Adapter, power negotiation does not occur.

Without delving into the deeper technicalities of how USB PD works, suffice to say that there is much more to this technology than simply providing a fixed power output. So as to protect the Power Adapter, iPad and cable, the Source (Power Adapter) and connected device must mutually agree power settings before power is applied. This handshake does not occur when using a standard USB Power Adapter.

Your iPad will need to be tested and assessed - and will very likely require repair. You would be well advised to visit your local Apple Store where the technicians can conduct appropriate tests and outline your options.

Genius Bar Reservation and Apple Support Options - Apple

If charging your iPad from 4% to full capacity in an hour, something is definitely amis with the iPad. Even with a USB PD Power Adapter, a healthy iPad battery cannot be charged at the rate suggested by this observation.

The Apple power/charge cable could be bad/faulty.