I have bad news... I can't reproduce it. Perhaps it does behave differently on 4K displays? Unfortunately I cannot test it.
The only thing I could think of is to try with gamescope. In Desktop Mode, you can do that by exiting Steam, opening a terminal, and running gamescope -e -f -w 3840 -h 2160 -- steam -tenfoot -steamos. This seems to work great for me... but to be honest, also running Big Picture mode without gamescope seems to work for me. You could give it a try and see how it goes.
Another thing you could try is to add gamescope -e -f -w 3840 -h 2160 -- %command% to the launch options of a specific game. But this might not be what you want at all. Personally, I wouldn't do this.
Another very reasonable thing you could do is just wait a few days and see if the developers are going to notice and fix the issue.