Clicking "Add MP3" adds ~1min of silence to the end of the file

[ouchmypockets]

If I am bouncing a WAV and I click "Add MP3" then the exported MP3 has ~1min of silence tacked on to the END of the file, while the WAV remains the correct length. So for example, if the project is 7m15s long and I Bounce To Disk, select WAV, and click the "Add MP3" box to the right, on export the progress bar will actually read "x:xx/8:15" and the WAV will be 7:15 long while the MP3 will be 8:15. However, if I export only to MP3 then that MP3 will be 7:15 long. It doesn't matter if the file(s) or project are 3min or 120min that exported MP3 always has almost exactly 1 extra minute of silence at the end (and the WAV does not).
We have found this issue to be office-wide, all of us operating the same version of PT and using roughly similar OS.
Is this a bug or have we inadvertently checked/unchecked some unknown box somewhere?