Thanks for your bug report. I have not been able to reproduce this so far. I tested both v7.78 and the current beta under Windows 10 and 11.
If you test this in a clean copy of mIRC, in an empty folder with no settings, scripts, etc., do you see the same issue?
Can you describe a step by step method that reproduces this for you? For example, here is what I did:
1) Run mIRC
2) Type //echo $sfile($mircdir) in the status window editbox and press the enter key.
3) Click on a filename.
Also, does this issue appear in the DCC Send dailog? It uses the same file explorer system dialog as $sfile().