Just started having this issue today. All previous versions worked fine on my main monitor. After today's update to stable version 0.34.115106 my game no longer opens on my main monitor instead it opens on my 2nd monitor. I have verified that the monitor I want it to open on is set to main monitor under windows screen resolution menu.
I will attempt to fix it on my end, but wanted to put this report in since the only thing that changed was the update to 0.34.115106. Just before that the game worked fine and opened properly.