From the promo material:

MIDI SuperTracks are MIDI tracks that can be added to a track or a style, and play like other MIDI tracks in a style. They are called “SuperTracks” because they are generated using a different engine than typical MIDI style tracks. Typical MIDI style tracks are generated from C7 patterns in the style, and repeat these patterns over any chord. MIDI SuperTracks use actual MIDI playing from musicians (similar to RealTracks in that regard), so are not based on patterns.

Selectable PPQ for MIDI Files

We’ve also added other new MIDI features, such as selectable PPQ resolution for export of MIDI files (from 120 to 1920 PPQ).

By default, Band-in-a-Box saves MIDI files with 120 PPQ resolution. This is about 4 millisecond resolution, which is very good resolution. In our listening tests, people couldn’t tell the difference between files at 120 PPQ and files at higher PPQ (like 960 etc.). But if you prefer to work in a DAW at a higher PPQ, you can now have Band-in-a-Box output at the resolution that you want.

Perhaps you need to choose a compatible ppq to ensure you can load them.

On the other hand it seems like the new engine has something to do with it.

Only time or Peter will tell.

John Conley
Musica est vita