I experienced this problem after 3 months of ownership. After much research with Tesla, escalating to their engineering team, and providing the ingredients for every product used on my vehicle, the problem was identified. The rear window trim is made with a stamping process much different than all other areas of the vehicle.
This is why the finish only changes on that area. The stamping process used cannot come in contact with a PH level higher than 13. I spoke with my local showroom where they shared this information from other owner complaints. (They have an internal case logging area)
I had taken my car to a touchless car wash a fair amount, it utilized simonez products. I used touchless as recommended in the car manual. What I was not aware of is that every touchless wash uses a high and low PH soap to remove grime and dirt. The simonez soap product (quest/mayhem) has a 13.XX PH level. Above the Tesla threshold that owners are not made aware of.
I was advised this defect was not covered by warranty but fortunately after a bit of back and forth, they chose to make a goodwill repair this one time with the caveat to only use products below that PH level as it would not be repaired again.
The recommendation from the advisor is to first ask the facility the list of products they are using and confirm the PH level before use. That really did not seem reasonable so I’ve purchased PH balanced products to wash the car at home moving forward.
Hope this helps clarify why this is happening for you all!