Change in plane and support structure should be caulked.

There will always be enough movement to make grout fail.

Indulge me in another q?

I just took a very close look at it.
It appears to have been grouted and then caulked.
Are both necessary?

Only caulk should be at the change of lane & support junctions.

It sounds like someone may have grouted the joint, and then gone back to caulk after the grout failed (and without removing the grout that never should have been there).

Thanks, brickeyee.

If you can stand it, one more, I pray you.

My new caulk job around the kitchen sink (corian, cast iron, overmount, originally done in about 1994 or so) which I've tried twice, increasing the drying time to 4 full days the second time, shreds/bubbles and comes off.

I checked the date on the red water-cleanup DAP Kwik Seal Plus Kitchen & Bath Adhesive Caulk tube and called them, and they say that there's likely silicone in the joint from glue or whatever. They advise using silicone caulk around the sink area.

Which should I use on the corian/tile backsplash junction:
silicone or water-based caulk?

"Which should I use on the corian/tile backsplash junction:
silicone or water-based caulk? "

If the area is silicone contaminated you likely have little choice but to use silicone, but even then you need to get the area as clean as possible.
Naphtha seems to work better than other solvents.
If you get the silicone level low enough new silicone caulk should work fine.

Its chief drawbacks are being a PITA to apply and clean up, a limited number of colors, and it is not paintable (not an issue in this application).

I have noticed a lot of poorly done tile joints lately.
Wall tiles sitting tight on counters leaving no real space for caulk.

Bathrooms seem to be even worse.
Wall tiles so tight against the floor of the shower you can only make a large cove of caulk instead of a narrower joint that is less prone to damage from cleaning (leading to the inevitable leaks).