Hi Victor,
Can you please give us a little more information.
1. What version of BIAB do you have?
For example is it BIAB 2017, 2018, 2019, etc.
2. Are you up-to-date with the latest build for your version of BIAB?
For example, if you have BIAB 2019, the latest build is #628. To find out what build you have... go into BIAB, click on Help, click on "About Band In A Box"... the build will be in brackets after the version. Something like "Version 2019(617)" -- this says that it's BIAB 2019 with build 617 installed.
Updates for all versions of BIAB can be obtained from the link below.
https://www.pgmusic.com/support_windowsupdates.htmOften, installing the latest update will fix the problem. I have BIAB 2019(628) and I use tempo changes a lot in my songs. When I render them to WAV, the tempo change is always there. Have you tried rendering to WAV to see if that works? If it does, it's possible to convert a WAV to mp3.
Regards,
Noel