This forum is for Destiny help. If you have Halo questions, you should direct them to the Halo community forums which Bungie is no longer a part of: https://www.halowaypoint.com/en-us/forums. My non-expert suggestion would be to do the level again on Legendary and see if it registers complete.
Hey Beastphantom, Bungie no longer owns the Halo franchise. Please visit this site for help: https://www.halowaypoint.com/en-us