To ensure that specific namespaces are used (i.e., treated as sacred) by Fedora 4 when doing a SPARQL update, ensure that the namespace prefix URI includes a terminating character. Otherwise system-generated JCR namespaces (e.g., ns001, ns002), and not the user-supplied namespaces, will get registered. Therefore, the following statement could lead to unexpected namespaces. F4 will register a system-generated namespace and you would not find "http://myurl.org" namespace in the system.
Therefore, the URI prefix in line 1 should have a terminating character, e.g., a slash or a hash.