Unfortunately I am unable to run Windows 10 with two displays at this time, so I cannot test this. It works correctly with Windows XP, 7, 8 though.
In the past, issues like this have been caused by graphics drivers and especially so after a new version of Windows has been released. If you update your graphics drivers to the latest version for your graphics card, that might help but otherwise you may need to wait for an updated graphics card driver.
That said, it is possible that Windows 10 changed the way that multiple monitors work but this would affect all existing applications.
If anyone else here is using Windows 10 and can test mIRC on multiple displays, please let us know if you can reproduce the above issue.
