To, date, no one, including Apple have NOT been able to isolate this issue.
No one knows if this is a hardware or software issue or a combination of both.
I, personally, suspect an issue with the iPad's underlying screen digitising hardware layer and an issue with palm rejection.
I have been using a thick drawing/writing, lycra material based drawing/writing glove with my iPad Pros for qute a few years now.
The glove isolates the electrical impulses of body ( in this case the palm or side of your hand ) from being absolutely undetected by the iPad's screen.
I have been using ine of these for years with my Apple Pencil and other smart Bluetooth stylii and have never had any issues with erratic or stray marks in my Apple Pencil/third party stylii compatible apps.
The glove is, also, suffificiently slippery to allow much smoother hand gliding across the iPad's screen ( with no hand/skin smudge marks on the screen ) for better drawing and writing.
If you wish to try one of these for yourself, here are, but two to be found on Amazon.
There are a few other makers of these iPad/tablet glove, as well.
The ones I use seem not to be available from Amazon, any longer.
Here is something similar, like these found on Amazon.
https://www.amazon.com/AKX-Rejection-Drawing-Graphic-Sketching/dp/B08R6PWKC9/ref=sr_1_33_sspa?dchild=1&keywords=tablet+drawing+gloves&qid=1611535160&sr=8-33-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExMFg5WFBZWFpCS1kmZW5jcnlwdGVkSWQ9QTA5OTA2MzMxRUVYUURUMlAwVU4wJmVuY3J5cHRlZEFkSWQ9QTAxNDY1NjMxVjg1NE45TVRXSUQ3JndpZGdldE5hbWU9c3BfYXRmX25leHQmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl
https://www.amazon.com/Timebetter-Rejection-Drawing-Sketching-Suitable/dp/B08DKGHYMS/ref=sr_1_21?dchild=1&keywords=drawing+gloves&qid=1611533128&sr=8-21
Some other glove makers, like these examples, found on Amazon, as well.
Best of Luck to You