As suggested by someone on this thread, I think it's the combination of both the hot sunshine and the windshield fluid drying out on the panel.
That's why I always make sure I wax that panel everytime I wax my CLA.
Although you keep your vehicle under the porch when not driven, but during driving, you could be using the windshield wash fluid during driving to get rid of bugs or dirt, and the florida sunshine is so hot, it literally bakes the windshield fluid droplets onto the panel within minutes.
Just as people would tell you to never wash and dry your car in bright sunshine, same idea, water-spots on your paint can be really difficult to remove, especially if it has baked into your clearcoat.
Changing the panel altogether is an (expensive) option, wrapping it makes sense too.