In Lightroom 7.3.1 and OSX 10.13.4, clicking the switch to turn OFF the B&W panel does not disable the Black & White Mix sliders. The photo changes to reflect that the panel has been turned off by showing the photo as if the sliders were zeroed. Unfortunately the sliders can be moved with no change to the photo but when the switch is turned back ON the changes made while the switch was off and the sliders were supposed to be disabled are applied to the photo. If you don't notice the switch is off you can really mess up your settings.
Screen clip showing sliders enabled with the switch OFF
I reset my preferences and the behavior was the same. This happens on all file types.
Lightroom Classic CC 7.5 was released today and should address this issue. If you don’t see the update in your Creative Cloud App, hit the [Ctrl/Cmd]+ [Alt/Opt]+[R] to force a refresh. Give the new version a try and let us know if it solves the issue.