Is anybody else having the same problem? Every time I start the game I have to change the display mode back to borderless and to my preferred resolution. This is really annoying... :/

Have you tried the "Exclusive" setting? Not sure of the technical difference, but on my machine at least it is visually identical to borderless.

None of which means that there should not be a direct fix for you issue.

For the record, both borderless and exclusive, and associated resolutions, are stable for me. However I am playing at the native resolution of the display panel, which might figure into the issue.

FWIW I had this problem but after a while - with no update to the game - the right resolution "stuck". Not much help, I realise, since I don't know what if anything I did to make that happen.

same issue here but only for resolution above 1920x1080 like 2k res. If i stick with 1920x1080 the resolution stays ok. If i put 2k resolution it resets every time i play the game. Don´t have solution for it right now.