I installed SemanticMediaWiki on MediaWiki 1.27.0-rc.1 and 1.28.0-alpha and the semantic namespaces (Property, Type, Concept) are not loaded when the user is logged in. Obviously it works with MediaWiki 1.26.
A practical way to see it is to go on Special:AllPages, but it is really a general issue: when you create a page in the Property NS as an unlogged user you can see it in Special:Recentchanges (e.g. "Property:Name"), and when the user is logged the recent changes display ":Name".
I also installed SemanticForms, and the namespace added by SF is added. SF uses the hook CanonicalNamespaces, but SMW modifies the global config parameter $wgExtraNamespaces in its register function declared in $wgExtensionFunctions.