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.
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I knew that haha but I didn't know forum was wrong, thanks for the link man!