Currently, security trimming functionality supports only Action methods which are synchronous. The good news is that support for Action methods returning Task<ActionResult>
will be supported in the next official release of Kendo UI due in the mid of next week. I will suggest you give it a try once it is released and let us know if the problem still persists.
Join us on our journey to create the world's most complete HTML 5 UI Framework - download Kendo UI