I remote desktop into a Windows 7 machine and I've also had this issue since 7.62. No idea how to replicate it, it just happens at random and it's extremely rare. Older versions of mIRC do not have this issue.
You are the 2nd person I know of to have this happen while using remote desktop on Windows 10.
I reported this back in my Win7/7.62 crash report. Guess I should have made a separate post.
Screenshot from 7.62 on Win 7:https://i.imgur.com/1wRDAEO.png