the issue you're describing, is very common, unfortunately.
are you using a screen protection? glass or plastic?
I meet same issue.
I use temperd glass screen protector.
When i hold it by hand, it is normally.
one month ago I replaced the protective glass film with a plastic one (brand Dipos, Germany, already successfully used years ago with Nokia 1520): the phone now responds correctly to the inputs on the screen, no more false commands as before with the glass film.
I found this good situation both with nougat and now with oreo.
abecedario, there seems to be different kind of plastic screen protectors from Dipos in amazon.de. Which model worked for you? 9H plastic glass or some other model?
Jarnos, I've bought this one, I hope to be able to upload the image. I've bought it on ebay at a lower prize than amazon (incredible but true, sometimes it happens!)