I can not change the "Search completion" setting in neither of these places:

• https://www.mediawiki.org/wiki/Special:GlobalPreferences#mw-prefsection-searchoptions
• https://www.mediawiki.org/wiki/Special:Preferences#mw-prefsection-searchoptions

Whatever I do, my selection is not stored. It always goes back to "Subphrase matching (recommended for longer page titles)". While there might be a reason for this behavior, it feels like a bug. If this is intentional the setting should be disabled or hidden.