Unfortunately, you are very likely one of many people suffering from an issue that is causing iPad Pro models to periodically “freeze” and become unresponsive to touch-screen input. There are many reports of this issue occurring throughout this discussion forum.
iPad Pro’s of various models, including the latest generation of 2018 11” and 12.9”, are suffering this issue; the problem is reported to affect at least iOS 11 and iOS 12.
For insight, here are just some of the recent discussions on this subject:
https://discussions.apple.com/thread/8588201
https://discussions.apple.com/thread/250030800
https://discussions.apple.com/thread/250021992
https://discussions.apple.com/thread/8396370?answerId=34214121022&page=1
https://discussions.apple.com/thread/8588201?answerId=250044102022#250044102022
Whilst there are many theories as to the cause (including those from regular contributors who are fortunate to have no direct experience of the issue), at present, there is no known definitive cure for this problem. Those that are affected remain hopeful that Apple will eventually acknowledge and resolve this issue.