I built a shower about 6 years ago, and did the floor/wall joint with a sanded caulk that matched the wall grout color. I spent yesterday digging it out because it had pulled away in several places, and there were several places that it had almost seemed to calcify - the caulk was hard and brittle, more like grout than caulk. The pulling away seemed more pronounced in areas I had used foam backing rod.
I'm wondering if this is due to water sitting on it after showers. If so, might it help to grout with the colored caulk, let that cure, then put a thin layer of clear silicone over it? Most of the joints are 1/8"-5/16".