The Imported MIDI file very likely has Continuos Controller data imbedded in it, like maybe CC7 Volume or other commands that were used to make the MIDI file sound right.

Band in a Box uses those same MIDI CC commands to control the Volume of BB Tracks.

You can take a look at the MIDI commands using the MIDI Event List.

I would first open the raw MIDI file in Realband, then use its editor to strip out all of those CC's, then Export the result as a Standard MIDI file and then replace the MIDI file imported into biab with the stripped file.

This is not a program bug at all.

And, you did not mention this MIDI file until now. That missing info can lead things far away from help or understanding.

--Mac