The top-level DSP Engine switch doesn’t turn off anything if all of the subsections are turned off.
The switch defaults to displaying in the “on” state so people don’t need to do an extra click in a confusing place, but that is just a display trick to save extra clicks.
Nothing is actually being turned on/off–on the inside that switch is just a shortcut that means “treat all of the subsections as disabled even if they are enabled”. But the actual enablement/disablement all happens at the subsection level.
Something else must have changed during your testing to make you hear a difference–I can’t say what it is, but there is another variable here you’re not accounting for. It’s not the switch.