Direct injection engines have this common issue. Just coked oil on valves and rings and occassional heavy accel will leave this deposit. Also what gets trapped in the exhaust system.
The revised stator plate might be nice so you don’t get the cold start 1-2 jarring shifts. My tranny shop also recommended me a different brand torque converter. I’m only at 75k so I don’t know yet what I’ll do.
https://gearsmagazine.com/magazine/8l90-changes-and-updates-they-are-not-created-equal/
There is a revised stator plate with a check ball for a fluid passage that prevents this.