I tried using File - Save Special - Export as MP4 (which is an awfully convoluted path, but the only MP4 option I found).
While it was rendering my destination location folder showed the "test.wav" file (temp file) in the image below (I had saved it as Test.mp4).
When it was done rendering this temp wav file was gone .. but there was no MP4 .. so I got nothing.
This type of un-intuitive behavior is why I get out of BiaB as soon as possible in a project. Just finding how MP4 was supposed to work left me frustrated .. then for it not to work, even more frustrated.
Hopefully someone else has a solution that works.
I tried ..
Description: After