The BIAB styles define the velocities for the instruments. If you generated the track using a style, the track should have the velocities as defined in the style. I've done quite a few modification of styles and rarely find the velocities the same throughout the song. Usually the 'B' part has higher values than the 'A'. You can check on this by editing a style or using the piano roll editor (among other ways).

There is a BIAB Opt|Preferences|Arrange setting to normalize velocities. I don't think it will make all the same velocity but comes close. By default, this is disabled.

R