Not the poster you're asking, but T-bone does this too and lies sideways with his belly out. He started getting spots a little under a year old. I asked the vet and she didn't seem too concerned. But you can use dog sunscreen. (Even the ocean creature safe human stuff can be bad for them.) We started doing that for the days he really lies out there baking.

He doesn't generally lie with his tummy to the sky out there. He prefers to let the black fur soak it up like a solar panel. He has a "saddle" pattern like a german shepherd.