Using MW 1.26.2, the steps that issue unexpected redirect are:

1. log out
2. attempt to view a page that has manually defined as not accessible without login by extension Lockdown, e.g. $wgNamespacePermissionLockdown[NS_INTERNAL]['read'] = array('user');
   • the following works: the login provided by the displayed system message loginreqpagetext (“You need to [log in] to visit the page”) succeeds with returnto=Internal:Page_name. OK
   • the following works unexpected: the login link provided on the top right results in returnto=Special:Badtitle not OK

Anything to circumvent this?

• redirecting always via to $wgRedirectOnLogin would be a solution but seems a bit crude