A RIMS tube during the mash will likely not result in a hot SSR. The heavy lifting for the water heating is not happening, and the RIMS is just doing short sporadic bursts of heat to keep the temp maintained. In other words, the SSR is OFF way more than it is ON, so not much heating up of the SSR.
During big temp raises and while maintaining a boil, the SSR will be ON a lot more than during a temp-controlled mash, so that's when your heat will start building.
During big temp raises and while maintaining a boil, the SSR will be ON a lot more than during a temp-controlled mash, so that's when your heat will start building.