I'm using Loki (though this might apply to all versions). I have a laptop whose screen is my primary monitor. Additionally, I have two external monitors, one to the left and one to the right of my laptop screen. This is how they're virtually positioned in my Displays settings, too.
The middle monitor is starred as my primary. However, almost all windows open in the left monitor anyway and have to be dragged over to my main monitor.
This happens when new applications open, when existing applications open new windows, and even for things such as the login screen (though I think that might be a separate bug, the cause may still be related).
Is there some way I can force the default monitor for things to happen to be my main screen, rather than the first screen?