Actually, in my experience, a lot of it depends on the original MIDI file type.

"Standard MIDI Files come in two basic varieties: a Type 1 file, and a Type 0 file (a Type 2 was also specified originally but never really caught on, so we won't spend any time discussing it here). In a Type 1 file individual parts are saved on different tracks within the sequence. In a Type 0 file everything is merged onto a single track."
https://www.midi.org/articles/about-midi-part-4-midi-files

It may be that simply saving the MIDI file as type 1 is all that is needed .. that said, BiaB does reserve certain channels for certain tracks. This may cause you to need to change some channel assignments.
Even if opened in RB, if it is a type 0 file all channels will likely open on a single track .. but in RB you can use Track-Extract Channels to Tracks to quickly correct.

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