I can confirm that this is not a magnetic problem. I removed my case yesterday, plugged in the charging cable and turned off the iPad for 12 hours.
I used GoodNotes for 3 hours on 100% brightness today. There was no problem with using the iPad with fingers or Apple Pencil until the display got warm.
So it has to be a heat problem. The recent open beta does not solve the problem.
A friend of mine has the same iPad as me (iPad Pro 10.5) but with cellular and he has no problems at all.
I used GoodNotes for 3 hours on 100% brightness today. There was no problem with using the iPad with fingers or Apple Pencil until the display got warm.
So it has to be a heat problem. The recent open beta does not solve the problem.
A friend of mine has the same iPad as me (iPad Pro 10.5) but with cellular and he has no problems at all.