The / is a valid character in Windows.
If you are not sure whether your filenames will end up with valid characters, use the $validate() function for the whole filename.
If you have more special characters that you want to replace with a special (other) character, uste the $replace() function for that piece of data.
Also see the FAQs on the Convert function and the possibility to create folder structures:
/t/8016/1
I would say: no bug.