Thanks for looking into this, however this behaviour is by design. There are some contexts where a DeleteFile() is queued and may not be processed immediately by Windows, resulting in side-effects if an application tries to access the file soon afterwards, so mIRC specifically forces an immediate move of the original file. This was implemented a long time ago to resolve issues related to DeleteFile().
While I appreciate you looking into this, the only real way to track this down would be to find a way to reproduce it by inducing a BSOD, since that seems to be the specific situation which causes it and which I have not been able to reproduce.
