i think thats because your custom onvolControl callback now overrides the onControl callback that was added by the ConnecttoModule command.
You can now add something like this
SimpleGain.setAttribute(SimpleGain.Gain, value); to your custom Callback. (you can see your legacy code at the onControl TAB / and by the way clean up :))
Oh, and dont forget to declare the SimpleGain via
const var SimpleGain = Synth.getEffect("Simple Gain"); in your init script ..