We have a 20-year history using a wide variety of Mamiya glass professionally- all pre-AF. I simply have no frame of reference for Mamiya turning out a bad model, much less any reluctance to make amends if a bad individual unit makes it through their QC.
I'd send the lens to Mamiya for evaluation, but be prepared for them to tell you there is no problem with it. While waiting for results from Mamiya I would evaluate your methods and the remainder of your hardware for the source of the problem. There is no sure thing on any of that, but experience has given me lots of reasons to respect Mamiya and their products.