Try redoing Forerunner. Looked at your campaign history and while it says completed its the only mission that doesn't have a score so that mission didn't record correctly. Try redoing that 1 mission and seeing what happens.
I have also complete Halo 1 on Legendary and seeing a similar issue with achievements. Lower difficulty achievements are stuck at 95% completion. In other Halo games, lower difficulty achievements would unlock after completing the game on Legendary.