Just installed the free trial, had the 720p issue, and search led me here. I'm using a second monitor with a custom resolution to make it match the 75hz of my primary display. Can confirm that setting the secondary back to its native refresh rate lets me properly change resolution settings in-game, but borderless windowed is set to 60 FPS no matter what.
Hey there dotbik. I'm glad to hear that setting that secondary monitor back to it's native refresh rate seems to help in-game, even if windowed appears to keep at 60 FPS. I recommend, if you haven't already, opening a ticket on our Support Site so that you can send over your System Files.