Hello,
i noticed this problem on 6.16 and i had a look at the page search where i found this
looks like the problem is solved with this version but i found a combination where we can see it.

If the file is been used atm (read or write access) and you try to see its size, you get a negative number.

To test this situation the best way is to have a loop reading the file, and maybe another mirc trying to check its file size (can be same mirc)

thanks for your attention