Single-walled stainless steel tanks are preferred precisely because of that thermally conductive property. That very characteristic is what makes precision temperature control in a tightly maintained water bath possible. Very low thermal inertia. Contained liquids respond very quickly to changes in external water bath temperatures.

In my own case I maintain water bath temperatures using a Hass Intellifaucet K250. Marvelous temperature control possible using this precision tool. In the winter I often set the Intellifaucet to, say, 68.4F to counteract the slight loss of solution temperatures resulting from the colder darkroom ambient air temperature. Single-walled stainless is what makes this possible.

What needs to be insulated is not the stainless steel processing tank (or tanks). It's the container holding the water bath in which they sit. That's the common reference point that you don't want to drift. I would never use plastic processing tanks. Too difficult to precisely adjust. Especially for color.

This particular behavior isn't a bug. It's a feature.