See mai.wikipedia.org for example.
I've noticed this on several other wikis in the past week as well. Presumably it is breakage due to the OOUI conversion conflicting with a common site customsation that various wikis have.
I don't think we need to support this override, but it would be a big help if we e.g. announce in Tech News what site admins can do to unbreak this as it's probably not obvious to them what to do about it, or even to know that it's a bug on their end, given that afaik these sites's aren't (intentionally) customising the filter itself (it's a newly added widget).