Hi,
it is really weird, I have exchanged the ESCs on the ports to see if the issue wanders with the ESC, but no it remains on the same port.
I then changed the port where the issue occured which was port M5 for the rear left motor, to port M4, but it remains on the rear left motor. For the motor marked as D in the GCS mixing picture, Autoquad seem to be sending different signals than for all others, and this is independent from the port used and the ESC/Motor connected to the corresponding port.
This is making me feel really uncomfortable and will stop me from flying the Autoquad till the issue is solved.