thank you very much for your answer. Having read it I decided to start the current build of Mp3tag to check the tagging of m4a-files I tagged with earlier Mp3tag versions - and indeed: They don't have a leading zero, either.
The only problem I have with this minor change of program behaviour is the following: I'd like to have the leading zero at least in the filename. Now, when I use the tag -> filename converter I don't get it properly done - the new %track%-string reproduces only exactly the content of the tag. An example: What I get for
%track%. %artist% - %title%
8/14. The Beatles - Eight Days A Week.m4a
This can be even problematic, because the slash is not a caracter that can be used on a Windows file system.
So far I haven't found an adequate variation of the %track%-string which allows me to have
- The Beatles - Eight Days A Week.m4a
although the number of total tracks is stored in the track-tag, too.