After having started it with one monitor and changing it to windowed it now seems to work regardless of whether my second monitor is plugged in or not. No clue what's going on there, but at least it works!

@magge I did not alt+tab

@Scout_More_Often I do sometimes have a second screen. I'm unsure as to whether windows thought I had it plugged in at the time, though the cable was not in at the time.

fullscreen

I'm unsure of how else to describe this. The game launches, disappears when I'm not looking, and when I alt+tab back to it stays up until I click and then displays the window below it. Does this on Steam, too.

Only have one monitor plugged in, if that makes a difference.