I had the same thing when I changed my plugs. Cylinders 4, 5, and 6. It got progressively worse going from cyl 4 to 6. There was even some orange color on the piston face in those cylinders. I was going to run the new plugs for a few thousand miles and then pull them and check again.
On other platforms, this is commonly caused by octane booster, but I've never run octane booster either.