Steps to reproduce:
- On https://en.wikipedia.org/wiki/Special:GlobalPreferences#mw-prefsection-rendering, set your skin globally to "Modern".
- On https://en.wikipedia.org/wiki/Special:ApiSandbox, select globalpreferences as action, select the first example ("Make a single preference non-global", which will make skin non-global again), and make the request.
Expected: The skin should use the local preference again.
Actual: The skin is still global, you have to reset it on Special:GlobalPreferences instead of using the API.
This also happens for other preferences, if you try this with userjs-* preferences there is no way to make them non-global again, as these are not on Special:GlobalPreferences.