Understand the different redirection behaviors when adding/removing a relation

Depending on the plugin configuration, the system redirects to two different URLs after adding or removing a relation:

• http://localhost:8080/nuxeo/document_relations.faces, which displays the Relations tab
• http://localhost:8080/nuxeo/default/nxpath/path/to/my/doc@document_relations?mainTabId=MAIN_TABS%3Adocuments&conversationId=0NXMAIN&tabIds=MAIN_TABS%3Adocuments%2C%3ATAB_RELATIONS which display a Page Not Found error

Why?