Hey marodilp,
Sorry you're running into this. I fixed a situation that could cause this on Windows a few patches back so if you're not on the latest update I would recommend updating.
If it is not too much trouble I would like to ask for some details: What triggers the window moving back to another monitor? Such as closing and reopening MiniMeters, the PC going to sleep, or something else?
And if you can consistently trigger the issue, could you please perform a Save As on the preset you're using and name it "Before" and then trigger the issue and do another Save As with the name "After". Then, quit MiniMeters, go to the "%AppData%\MiniMeters" directory on Windows or "/Users/[your name]/Library/Preferences/MiniMeters" directory on macOS and send me the "last_run_log.txt" and the two presets you saved to contact@minimeters.app?
Thanks,
Joe