Thanks for your bug report. I have not been able to reproduce this issue yet.
Are you using anti-virus software? If so, which anti-virus software is it? If you disable it, or add an exception for mIRC, does it resolve the issue?
If you install a clean copy of mIRC to an empty folder in your Windows Documents folder, using the "portable" option in the installer, with no scripts or addons, and then test this copy of mIRC, do you see the same issue?