It's the auto-assignment of CC controls on faders 7/10 of midi tracks.
For example:
I want to control the Fader and Pan of an audio bus.
I configure it to receive from channel 16 CC 7/10 respectively.
Everything works fine, because when I assign it in the Audio Bus, the midi tracks stop receiving that CC control by default. They lose the default CC reception.
Now, if I remove or change the CC reception on the audio bus, the midi tracks recover it, and the loop arises again.
Conclusion:
The midi track faders should work like any other fader element in Qtractor. They should not receive CC controls if they are not expressly assigned to them.
I think I've finally found the problem.
It's the auto-assignment of CC controls on faders 7/10 of midi tracks.
For example:
I want to control the Fader and Pan of an audio bus.
I configure it to receive from channel 16 CC 7/10 respectively.
Everything works fine, because when I assign it in the Audio Bus, the midi tracks stop receiving that CC control by default. They lose the default CC reception.
Now, if I remove or change the CC reception on the audio bus, the midi tracks recover it, and the loop arises again.
Conclusion:
The midi track faders should work like any other fader element in Qtractor. They should not receive CC controls if they are not expressly assigned to them.