See #21854: Mixer "solo" doesn't apply in audio export (though mute does). I believe this is actually by design - think of that setting as being more for temporary overrides, and in many cases that's what you want. But of course, there are other settings where you might really want the solo exported and for whatever reason don't want to use parts, which would be the normal way of getting individual parts exported.
Comments
See #21854: Mixer "solo" doesn't apply in audio export (though mute does). I believe this is actually by design - think of that setting as being more for temporary overrides, and in many cases that's what you want. But of course, there are other settings where you might really want the solo exported and for whatever reason don't want to use parts, which would be the normal way of getting individual parts exported.