Which in XML is an information_only property which has to call some command on my plugin.
However, it is not actually called, which is seen from the debugger. In python returns some old value, and the target command seems to have been called only once.
What is the correct way to force real execution of my server-side (plugin) command and getting its result?
That doesnt make much sense, the client control what the server does. If you are running code server side, you can just access your server side filter directly.