So, I tested some older ones where I knew it wasn't a problem, and it was still there so it wasn't DOPus code....sorry!
I did some more testing, and It has to do with windows and their 'scaling' in the display. I've always had my main monitor at 150% and secondary at 100% (because it's an old low DPI, but high resolution monitor). It's never been a problem for years! (other than when dragging windows across, it does weird redrawing), but it seems some update must have borked it. If I set both monitors to 150%, DOpus has no problem registering the X/Y coordinates.
It's strange because windows file explorer has no issue, but I can confirm it's a bug when 2 screens have different scale percentage settings.
I think I'll research that and see what's going on from the Windows side. Maybe this might help you if it was a new windows update messing with display settings. I'm not sure if others are having that problem.
Thanks for helping out!