NS_MAIN is missing from NamespaceInfo::$canonicalNames which means that NamespaceInfo::getCommonNamespaces() does not return the correct result.
Checking out the WIP patches from T200938, after this change blocks including the Main namespace are displayed in Special:CentralAuth as a separate bullet point (Main).
Before, it included Main as one of the ...and #n local namespaces.
This change potentially affects the SemanticMediaWiki and ManageWiki extensions. I have raised bugs with the respective developers https://github.com/SemanticMediaWiki/SemanticMediaWiki/issues/4290 and https://phabricator.miraheze.org/T4710.