I switched to Windows 10 from Windows 7 last month, and one of the issues I have been noticing is that when mIRC is minimized to the tray, it seems to disappear. This usually happens overnight, when the computer is idle. When I return, the icon is missing. However, when I view the Task Manager, It shows it is still running. So, I end up having to kill the process and then restart it.
Today, I downloaded the latest mIRC beta and will update you in a few days if that version also suffers from this as well.
Version: mIRC v7.55, Windows 10