This could be related to the MIDI file format being used. MIDI Type 0 has all channels on one track. MIDI Type 1 uses different tracks for each channel.

Use File > Open Special > Open MIDI file. The shortcut keys are the letters S S 9 <Enter>

There is more description about the two types at this link:
https://www.sweetwater.com/sweetcare/articles/what-difference-between-midi-type-0-midi-type-1/


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