The autos do what is called "rev matching" on the downshifts when in M or S mode and may be what you are feeling when it does its own inadvertent downshifting. Still, it is def not normal for it to do it on its own UNLESS it is in Sport mode. I would ask the dealer if there are any updated calibrations for all the control units on the car (ECM TCM BCM). They are usually applied together, at least for the ECM and TCM....
If they are no updates, remove the positive battery cable and let it sit overnight to drain residual power from the control units. This is overkill but will ensure complete reset. The trans is an "adaptive" unit and the TCM "learns" shifting patterns and stores them. It may have stored an incorrect shift procedure or simply botched it when storing....
