That's normal. The tone control in the circuit is a pretty weak low-pass filter, same as the Cornish G-2. The range is narrow so while shaving off a bit of treble, it more so just acts like how you noticed, a secondary volume control. If the main volume knob is run a bit lower, you can can hear a bit more 'sweep' out of the tone knob...barely. But overall it's a bit useless. It's worse in the G-2 circuit, and better in the SS-3 circuit where it's paired with a high pass filter and you can actually shift the tone around.
If you want even more ridiculous...bias knob on an external pot and the Cornish buffer circuit in front.