From my experience in auto refinishing, I'll tell you one thing, paint is a riddle sometimes. The answer is hidden within. I saw a paint problem on VW's painted a similar color as your truck. The clear would become milky and then peel off. VW paid to refinish the affected vehicles regardless of mileage.
At first glance, it looks like someone took a dual action sander with 1000 grit and sanded down the clearcoat. As paint is sanded, each layer becomes exposed and each layer has different characteristics. When the clear is sanded and the basecoat exposed, that ring is the first thing you'll see. Sand further and the basecoat quickly yields to a primer layer. For 4R's I beleive the primer is a light grey or white.
That whiteish "ring" is a strong indicator of reparwork.
I know you said that you've never had any repair work done, but possibly a repair was made, most likely at the port of entry. It looks like a spot repair showing signs of age. However this is just a guess because I'd really need to see it in person to tell exactly. A spot repair is used to repair a minor blemish on the surface, like a small scratch. To make a long description short, the inner-most area is sprayed with basecoat (color) and a slightly larger area sprayed with clear. However , the entire panel is not refinished, instead the reparied area is then polished to blend in to the underlying area. This type of repair is not going to hold up over time especially in areas of the country with lots of sunlight.
Has it ever been polished with a rotary buffer? It could have worn down the clear in that area and the sun did the rest.
Can you describe what is happening to the surface, it looks like the clear has lost is gloss. Is this correct? Any change in texture? Can that ring be felt by rubbing your hand over it?
Take it to a local body shop, not a big, high production shop, but a decent shop where you can talk to the guys without all the hype of fancy waiting rooms and damage appraisers who dont know anything about repairs. Good luck.
