Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
I’m on the iOS 14 Beta and have the same issue. Manually locking the screen before closing the lid works for me. Pretty annoying though. I hope it’s not a typical gen 1 problem...I reported it to apple
 
Same issue here. iPad Pro 12.9” (2020) on iPad OS 13.5.1

Have tried everything, issue persists every 3-7 days.

Have a case ID and they asked me to restore the iPad to factory settings, which I did, and it didn’t (no sureprise) solve the issue. Have a call scheduled on Thursday with them. Cannot get down to a store so if I have to replace will be a long wait...
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.