I’ve had repeated issues with Halo 3 not showing levels as being completed on the Master Chief Collection.
This is usually a problem encountered during Co-Op. Recently a friend of mine and I played through Tsavo Highway on legendary. We completed the level and he got the achievements and points for the level (he was player 1) but for some reason it still showed as un-played for me. Other levels showed as completed just fine, but Tsavo highway has now done this twice. It also occurred on previous levels like The Covenant, but those bugs were fixed.
Same problem here but with cortana on solo, it was painful and it really sucks that its not showing as completed, no checkpoints, no skulls, nothing