Although I cannot be 100% sure this is the or a potential cause, I can reproduce this broken behavior.
If you make a new project in Xcode and add a settings bundle, it will work 'out of the box' - and you'll see the sample settings populated in the template. They have String, Checkbox, and Slider.
If however you add a multi-value option, the settings bundle will disappear. If you remove the multi-value option, clean the build folder, and redeploy, you should see the sample setting controls again.
So this is possibly either a change in how multi-values are supposed to work or a bug. I guess the workaround would be to not have multi-value options in the short term but that seems a little user-unfriendly for a shipping app.