This maybe an edge case but if there's something underlying actually happening I thought I'd at least let you look at it. I found while I was worked with a powerful app named Rename Master. The developer of the app said the issue was not in his product (in my professional opinion I think it is), so let the dart tossing begin!
I'm using build v2.90a and have used many earlier versions throughout the years. It's always worked fantastically well (as always). I recently decided to take multiple live music discs and combine them into a single folder. mp3tag makes it extremely easy although the leading track numbers for the file names didn't change (thus the need for Rename Master. In this case, I added MP3s from a single Pearl Jam concert on three (3) discs to a new folder named simply "2016/04/16 Greenville, SC" The tracks were named as follows:
There are 35 tracks total. I used the Auto-numbering Wizard, so all the tracks tags were 1/35 through 35/35. I then renamed the Album tag (removing Disc 1, Disc 2, and Disc 3) so they all were simply "2016/04/16 Greenville, SC" and changed the discnumber tag to Save. All is good. I started Rename Master, which I downloaded from http://www.joejoesoft.com/vcms/108/, and first used the app to remove the leading pj160416d3_ from each MP3 file name. I then sorted by track tag. For some reason the 32 track tag did not appear at all (it was blank), so it was at the top of the list. The developer said that is NOT a bug on his side. I did this for again for 8 other concert and 32 failed to appear every time. Is it possible that mp3tag isn't properly writing 32 as track tag? I've got screen captures.
FWIW, There's a workaround in Rename Master that let me move the empty track number file down to the 32nd position, so I could rename the files 01, 02, ... 31, 32, 33, 34 and 35.