Thanks for your bug report. This issue is often reported, however I have never been able to reproduce it myself. Were any of your other mIRC settings affected? Do you remember if you were doing a full or upgrade install? Did you change any of the default install options (such as backups, shortcuts, etc.)?
No other settings affected. I've had this issue on a clean install of mIRC v7.44 as well as a mIRC folder I copied over from my external backup HDD. The only thing I changed was the default folder (which I use C:\mIRC instead of the default path chosen). During an upgrade, I usually choose "save to backups" and "automatically check for updates" options only.
My OS is Windows 10 Home 64-bit.