List of steps to reproduce (step by step, including full links if applicable):

• Go to global preference on any wiki, uncheck "use legacy vector", check the globalize checkbox before.
• Go to local preference on a wiki, adjust any local preferences (especially something only exists locally, like options on "gadgets" tab)
• Save your settings and, hey presto! You go back to legacy interface involuntarily!

What happens?:
The main interface of the site went back to legacy vector skin, regardless the "use legacy vector" option in global preference unchecked.

What should have happened instead?:
After saving local preferences, the main interface stays at new version of vector.

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc:
Almost appears on every language-specific Wikimedia sites (i.e. doesn’t appear on Meta, Wikidata, Commons, MediaWiki) on all common browsers