Sorry to hear/see that, gotta be frustrating! Did you remove or bypass your cat? If not, have you tried using a different fuel brand/source? Can you tell if your car is running too rich?
Does that part of the car get more exposed to sun or do you park it somewhere where automatic sprinklers hit the rear or maybe you're driving through something on a regular basis?
Would it be possible to tell if the clear coat wasn't applied evenly back there? I don't know if UV damage would clay bar off.
Also, is it my bad eyes or an illusion that it looks like there are two column like areas on the rear bumper cover directly under the two inner parking sensors where it's less yellow - like the yellow was flowing down and got redirected by the sensors?