Tuned and now gearbox issues :(


I have a 2018 prefacelift c63, intake, bigger turbos, fuel pumps, down pipes, ecu tune, tcu tune, the car runs fantastic on the boil, but part throttle or low rpm high gear low boost in getting what seems to be clutch slip.
The trans has been serviced and every thing looked clean, no clutch material no burnt oil, infact for its 1st gb service it looked amazing. Oil filled correctly.
the only codes I'm getting are :
the occasional [lean bank two down stream of the cats) and it only throws this at low rpm or idle every now and again. But I have no cats ??? Fueling checked and it's not running lean according to the primary o2 sensors.
P061B
Torque calculation of the control unit has a general malfunction
Has anyone experienced this before ? The tuner is working to fix it but they say they have never seen it before. The tuner is a popular m177 tuner I don't want to name, names as they seem to be doing the best to help
If it needs clutches ill get some, just be pissed if i do that and its still there, I can't see it being the clutches as higher up in the rpm no slip at all low rpm and low throttle input it slips a bit.
Valve body rebuild? Could a solenoid be lazy ?
zero slip codes in the box yet it is slipping.
any help would be greatly appreciated






