Exporting to MP3 with the wrong # number

I’ve been using Audacity with Windows 10 for a while and not had an issue exporting to MP3.
I narrate audiobooks and use the - 015_014 Chapter 14 - method to name files as I export them.

Usually the files are ordered correctly in the folder when I’m done.
Just recently, I’ve noticed that the # number (which I can’t change) is incorrect.

010_009 Chapter 9 has #8 when viewed in the folder.

Others have done the same thing. It was all fine until Chapter 7 and beyond.
I was using a slightly older version of Audacity (the version behind the newest one) so updated it to 3.0.5 and the file is still doing the same thing when I export to MP3.
I hadn’t wanted to update mid project as so much is different about the new version.

Is there some metadata I’m inadvertently adding without realising it? And why would this have crept in now?
I need the order to read correctly before uploading everywhere.

I’d really appreciate some suggestions as to how I can fix this.

I don’t know what’s going wrong but you can edit the embedded metadata with [u]MP3tag[/u] (and this is done without “touching” the actual audio).