Hi! Mosaico takes monitor info from Windows, so if it considers the two monitors as if it was just one, probably something is set this way in Control Panel or Video Card options. Please try to look there if there’s any options like “Extended monitor” or “Surround extension”.
If everything seems to be OK, please try to restart Mosaico after you switch monitors, and see if it works correctly.
Which version were you using before updating to the latest one?
Positioning with the J-Pad or keyboard shortcuts have the same problem, or they work correctly?
If you need a trial extension please contact me at firstname.lastname@example.org and I’ll provide one. Thanks!