Not extremely common but it's happening. Just had mine replaced (March 2019 build) but it was a fight. My grille was yellowing with only a 1000 miles but I decided to hold off for another 5000 so it would get worse, therefore more noticeable. My dealership was hesitant to even try because they hadn't seen it before, thought it was a chemical stain from my doing, my service advisor has a 2018 white JLU and his isn't yellow, and in some lighting it was hard to see the yellow. I don't blame them, but just be respectful and polite. In the end, since Jeepcares had dealt with this before, I ended up reaching out to them and they were able to get it replaced for me. It's something to do with the clear coat yellowing which I've seen before on cars that have been repainted, like hot rods. It's like they don't have enough UV protection mixed in the paint. I'd be willing to bet other colors are yellowing but you can't tell because of the color difference.