If running Sigil on one monitor (say the LG) and moving the app by dragging to another monitor (the other LG) then when you go to the top Sigil menu and click does the menu show up where it belongs or on the original monitor?
See Vanguard3000's original bug report here to see what the bug looks like.
https://www.mobileread.com/forums/sh...7&postcount=12
You could try repeating that test with the Acer pair as well. The bug requires the monitor edid blocks to have identical serial numbers in them.
If it all works then your Monitors all have different EDID blocks and can not be used to test with.
If you can recreate his bug, I would be happy to walk you through testing with the debug Sigil plus the new qt qwindows.dll plugin to see if it fixes anything.