I don’t know if any of you fixed this, but this may help someone else in the future. I had the same problem with LB opening Startup Screens on wrong monitor. I tried all kinds of fixes and options, but to make long story short, I eventually opened Big Box and it opened on wrong monitor as well. I went into BB general options and found that my display that was identified as #2 in Windows’ Display Settings was identified as display #1 by BB. As soon as I switched over to display #2 in BB options (Display #1 in Windows’ Display Settings), both my LB Startup Screens and BB opened on the monitor they were supposed to.
I’m guessing that because Windows’ “display #2” was my original single monitor display for LB/ BB, when I added the 2nd monitor that became Windows “display #1”, LB/BB did not sync w/ Windows Display Settings and just added that monitor as a 2nd display thereby creating a glitch in the matrix. I hope this makes sense and helps someone.