Issue with files GP and MuseScore

I play the intermediary on behalf of a guitarist on a French forum. He tries to import a guitar score at
three voices created with GP5. But it only gets two voices after import with MuseScore.

I never even noticed this (I use mainly GP for importing rock music or other)

I did a simple test with three measures.

With the gpx file, MuseScore import one voice, the first: test gpx three voices.gpx

With the same file, exported in gp5, MuseScore imports two voices, the first and second (and therefore: no bass) : test gp5 three voices.gp5

With the XML file, I get three voices. More exactly, I get the tablature (and with problems with multiple tab clefs). By removing these clefs, and adding a standard staff (linked, or copied-pasted) the result is correct. test xml three voices.xml

Where is the problem with these GP files?

Will there a trick I'm missing? Or limitations are they known for importing a maximum number of voices aside MuseScore (I'd be surprised of that), or aside GP for the export of these voices?

Thank you for your lights on this point.