Hi,
I used to have this problem (often but not easy to replicate/provide specific scenarios) in my iPad pro 10.5;
Even in iOS 12.2 the problem didn't go away;
But after some googling on seeing this thread(I had resigned to live with it before that), I switched on the "touch accommodations" option in settings - without changing any of the default sub-options. This seems to have fixed the problem - at least for now. It's been a few days and the problem hasn't recurred. Will report back if this recurs.
Has this already been tried on 12.9 and found not to work?
Yup, improves but does not completely eliminate the issue for me.