Admin Rights UI : rights of local users/groups are not found if prefixed with the wiki name

Scenario:

• subwiki "foo" with local users "XWiki.TestUser" / groups "XWiki.TestGroup"
• these users/groups have certain rights assigned (as objects of class XWiki.XWikiGlobalRights" in the foo:XWiki.XWikiPreferences document). E.g. "XWiki.TestUser" has "allow" for "Admin"
• the users/groups are listed via their full name, i.e "foo:XWiki.TestUser" and "foo:XWiki.TestGroup"

In that case all the right checks are ok, but in the Admin UI, Section rights the relevant rights are not shown. I.e. there is no green "check" symbol in the "Admin" column for the XWiki.TestUser displayed, even though the user has that right.

This problem can only occur if the right have been granted to the users in some rather old xwiki version (in my case it is 2.1.1) and then migrated to 5.x, because in newer versions - when rights are granted -, local users are only references by their "local reference" (without the "foo:" wiki-prefix,) which somewhat makes sense as they are only defined in that wiki.