Hello MoinW,
Thank you for using Apple Support Communities.
It appears that you are having an issue with using the Apple USB-C to AUX converter with your iPad Pro. The same adapter works fine with other devices. You have made a good start on troubleshooting by testing the converter with other devices. Now, let's check other USB devices with your iPad Pro. You can take a look at this article for guidance:
Using USB accessories with iOS 11.4.1 and later - Apple Support
If you are not able to use other USB accessories, then it would indicate that your iPad will need service. This article will provide guidance to get service for your iPad:
iPad Service and Repair
Best regards.