I investigated the issue by reviewing the source code and believe I have identified the bug in macros.vm. The following code considers anything without an avatar that is not an existing group as a user without an avatar.
#if ($profileDoc.getObject('XWiki.XWikiGroups'))
#set ($url = $xwiki.getSkinFile('icons/xwiki/noavatargroup.png', true))
#else
#set ($url = $xwiki.getSkinFile('icons/xwiki/noavatar.png', true))
#end
I created XWIKI-22754 for the issue…