Per the manual, this can be affected by MIDI setup  Start here

https://usermanuals.finalemusic.com/FinaleMac/Content/Finale/ID_MENU_FILE_EXPORT.htm?Highlight=export%20mp3

Hmm, I looked at the Finale docs and nothing jumped out at me that .mp3 export is a somewhat flexible or dependent on some condition. 

I could be wrong here, but if I recall correctly, you should take a look in the MIDI/Audio menu.

* Is “Play Finale Through MIDI” selected?

Export “straight from Finale” as MP3 is only possible when playing Finale through MIDI.

If you play Finale through audio units, then you have to, afterwards convert the exported audio file to MP3 format, e. g. with iTunes.

Thanks. You're right that's indeed the cause. Devil is in the details.