I do not believe in mysteries.

In this type of models, the rudder deflection creates a side force that makes the fuselage yaw by reaction against the flow of air.
Then, one wing presents a higher AOA to that flow than the other wing, due to dihedral, lifting more and banking the whole model.
Then, the circular motion starts, and lasts as much as the bank.

If the model cannot turn to the left, is because the deflection of the rudder does not create the lateral force necessary to yaw the fuselage.
If that is true, and all the angles and lateral balance are symmetrical, then a lateral force in the opposite direction must exist.
If that opposite force has nothing to do with rudder deflection, then it must yaw the fuse to the right when rudder is neutral, which is not happening, according to the description.

Fly the model away from you and watch for fuse yaw for left, neutral and right rudder deflections.
Also check rudder alignment with vertical tail, after trimming for hands free straight flight.
Please report your findings; this is very interesting.