It looks like there is a mismatch in the feature flags when we changed the name from "sandboxVisualsDisabled" to "sandboxVisualsEnabled". The name change is already in the pipeline but isn't all the way through yet. For now it works if you specify both flag names:
?sandboxVisualsEnabled=false&sandboxVisualsDisabled=true
I am sorry for the confusion. Soon this will no longer be necessary.
Take care,
Bashir