I’ve completed Forerunner in Halo 4 on legendary but did not receive an achievement nor does it show completion on way point, any suggestions?
The same thing was happening to me with the Halo Combat Evolved Anniversary mission completion achievements, like I have only gotten the achievement for the Pillar of Autumn, but I sorta got it figured out. I played some of the missions on the playlists and I got some of the achievements, but after I finished 343 Guilty Spark, the achievements stopped showing again. So I would say try completing the mission in one of the Halo 4 playlists. Hopefully that helps.
The playlists have always been buggy, if I were you just avoid the playlists and just manually load up the missions and redo them to unlock your achievements.