I completed every mission legendary solo over a year ago and still haven’t got the achievement, i’ve checked on waypoint and on my xbox and I it says all 15 are done but it’s stuck at 73%
Think this may be one of those weird bugs where you ended up disconnected from Xbox Live during one or more of the missions. Unfortunately, I believe the only solution known is to replay through all the missions to see if the achievement will pop. Similar issues occur in the Master Chief Collection, bad luck sometimes.