I had the same on a mk6 Golf GTD - car was bought new and it came from the factory with with a misaligned rear bumper in exactly the same location as the OP’s (offside, where the bumper meets the rear quarter panel). A gentle persuasive thump with the outside edge of my fist sorted it!
After correcting the misalignment, it stayed perfectly aligned for the next 4-5 years that I owned the car.