On https://meta.miraheze.org/wiki/Special:ManageWiki/settings, the anti-spam tab is the default tab.
When the tab is selected, it should go white but when you first navigate to the page the default tab shows as grey as if unselected.
The method is thought to be the same as MediaWiki so it is believed to be a MediaWiki bug. You can find the code at https://github.com/miraheze/ManageWiki/blob/master/includes/specials/SpecialManageWiki.php#L107-L121
https://phabricator.miraheze.org/T5026 says this is an issue with MediaWiki. That is all I know.