You've hit on the problem that I have with BiaB and that is the underlying way it works. The manual does not, or doesn't seem to, go into what's going on underneath the interface.
In this particular case I see a midi file being interpreted by BiaB to detect the chords. The tempo and the key signature in the midi should be able to select an appropriate style to at least to start with and, when the song plays, the movement along the bars would be in time with the midi file. After all, that is all that is playing at this time. If you could explain to me what is happening in BiaB that sometimes, not always, this lock step with tempo is not the same. That way I could sort this out for myself.