The expected behavior is that PeerMark grades are not passed back to the LMS grading area. This is because PeerMark assignments are treated as part of the main paper assignment, and the LMS grading area only pulls grades through from a paper assignment, not its parts.
Note: All PeerMark related grades will only be visible within Turnitin Feedback Studio when logging into the Turnitin website directly via turnitin.com / turnitinuk.com.