Mine just started doing this but way worse! The gap is huge and I can clearly see the clip that is not clipped in correctly. Did you get this fixed?
My passenger's side where the rear quarter panel meets the bumper is the same as what you're reporting. I can see the clips, probably a 1/4" gap. Only problem is, I purchased my car in August and just came across this thread now. Wrote it off initially thinking it was normal for Ford cars. Have an extended warranty 8 yrs/100,000 miles, so I'm gonna call and see what happens. I can just see them trying to blame my suspension and exhaust mods for panel alignment issues.
Apart from replacing my puddle lights they haven't been eager to fix things or enter warranty requests in to the Ford system.
My 2015 also had alignment issues. Not as bad as some, but still not acceptable on a new car. Took 4 trips to 2 different dealers to get it 95% perfect. It soured me so much on Ford, considering my previous Mustang had issues like the trunk the not closing, after the warranty ran out. It was bittersweet but I traded in my Mustang last month for a WRX. Overall I’ve enjoyed my time with Mustangs, but after seeing the 2018’s still have the same problems, I’ve decided to move on. On paper the Mustang is faster, but the WRX at least to me feels just as fast, plus I have a usable back seat, and Eyesight. Unfortunately I don’t think I’ll ever return to Ford. When I contacted customer service back in 15, I got such poor responses from them, like they couldn’t be bothered. I hope Ford gets their shit together.