So, I'm having what sounds like similar issues and after some testing have what may be some useful info. So my layout is 3840x2160 monitor on left as primary running at 150% UI scale and my 1920x1080 on the right with a UI scale of 100%. In Big Box, I have no marquee screen and my main screen set to screen 2 (the ID of my smaller screen). When I start BB, regardless of which monitor I run the shortcut from, it opens on my 4k monitor at 0,0 with a resolution of 2880x1620.
If I virtually move monitor 2 to the left of my primary monitor, Big Box opens on it properly at 1920x1080, but still seems to be running at 150% scale. If I go back to the original positioning and set the smaller monitor as the primary, Big Box opens exactly how expected on it (100% scale, 1920x1080 resolution).
It also works the same as moving the second monitor to the left if I move the second monitor above the first, instead of to the left of it.
So I just thought knowing that monitor positioning has a lot to do with behavior may help with troubleshooting.