It seems that XWikiPreferences has an DocumentSheetBinding object with value 'ClassSheet'. This makes the XWikiPreferences page, in view mode, to look like a regular class instead of using the AdminSheet.
I don`t know what adds this object since the Administration application does not seem to have it by default. Must be some component (xwiki-platform-sheet?) that attaches it.
Workaround: Remove this object or replace it's value of 'ClassSheet' with 'AdminSheet'.