I have not seen this kind of mp3tag behaviour myself on any media file format. However there have been similar reports in the last that were confirmed and corrected when files were found to have some kind of error. See this previous thread when making a change to the cover art also changed the reported song length.
The tag information is only contained within the file header, this does not affect the actual audio details of the file itself. The length should never be affected by a metadata update.