I have a new bug since few weeks, did not managed to solve it by myself so here are details about it:
drag and drop audio files to MP3tag
select all files
add track numbers using the build in option "Autonumbering wizard"
select "Tag->Filename" option and set it up the following way:
%track%_%title%
... here is the problem, for example if the first track (out of nine) has title "trackone", it should give "01 Trackone" as result, but instead gives "0109 Trackone" ... got it ?
If you only want the track number use as
format string: $num(%track%,2)_%title%
Edit: Some further explanation:
Using only %track% would actually lead to 01/09 ... if the / would not be an illegal character for filenames. So that is cut out und you get 0109.
The $num() function transforms the string 01/09 into a number which means it stops at the first non-numeric character, the /.
The 2 in $num(%track%,2) sets the padding.