If he grouted the corners then covered the grout with a thin smear of caulk, the caulk could just be telegraphing the cracked grout. The issue is not if the grout was cured prior to the caulk being smeared over it. The issue is that there should be NO grout in the corners at all, just caulk. At a minimum, if the corners do get grouted, the grout should be raked out deeper than the joint is wide, then caulked. Changes in plane are where differential movement occurs. One wall moves independent of the other, or the edge of the tub flexes away from the wall when the tub is loaded with water, etc. Grout is rigid and cracks when movement occurs. Caulk is flexible and flexes when movement occurs. And I know, I know, he does this all the time and this is the first time it's ever happened to him. Uh huh.

Since the cracks are purely cosmetic you can use caulk to cosmetically fill the cracks. Do a small test to make sure the caulk visually meets your goal.

Carpenters don't caulk stuff. Painters do. I imagine sink installation contractors have caulking expertise, too. Your siding should have been completely primed on all sides and any cut edges should have been primed or sealed prior to installation and certainly prior to application of any caulk. If your corner boards are, in fact, treated material, you may have trouble getting paint to adhere well.

In my experience, it takes about a year for everything to "settle" as Johnson Brothers said ^^^^. That means you'll experience caulk separating during that time. It's happened to us in every single house we've owned that we built or remodeled. It's not a huge deal, just a bit of an eyesore. Our agreement with our builder was that he'd come back after 6 months to recaulk and he did. It's fine now except in a few places. Still not that big of a deal since I'm pretty handy with a caulk gun.