Tempo does not get saved to a wave file.
Use the native format or move to RealBand.

Realband will allow you to save multiple audio tracks as an seq file and it retains the tempo.

Unfortunately I think you'll find most programs do not include a tempo map when saving as a regular wav file. The exception may be if it is saved as a loop .. these can have the tempo stored in the file's 'header'. But these cannot account for tempo changes.

I think RB will be your best bet.
Open the BiaB file in RB.
Render it to an audio track (see note), save it in the native seq file format. Than add additional audio or generate new tracks at will.

Note: if you pay attention to the options available when using Audio - merge audio and DXi to stereo .. you'll notice you can merge it to a 'track' (in the current project) or to a 'file' (in wav format).

Depending on the use this can be a real time saver.
If you do render it to a file, you can always select a new blank track and use 'Usio Import' to import the wav file back into the .seq project and it will line up with the tempo changes.

I do not work here, but the benefits are still awesome
Make your sound your own!