Hm,
After several more scenarios I revealed the strange, may be, behavior of that extension method.
We should use <% Html.Form("MyController", "MyAction") %> instead of <%= Html.Form("MyController", "MyAction") %>,
Because the helper method Form(HtmlHelper, string ControllerName, string ControllerAction) is writing directly onto the Context, instead of returning a string. So when we use the <%= %> syntax, it will first write the output to the current Context, and then will output the type that is writing - in this case System.Web.Mvc.SimpleForm.
I think this is a strange behavior, because no other helper method (like Button, Hidden, etc.) is writing to the context, but is returning a string.
Regards,
Anton