I just finished the last mission of the campaign on solo legendary but I diidn’t get the acheivments or Mark VI!! Are you kidding me!
Did you watch the prologue and the epilogue on Solo Legendary?
go to your comendations and check which ones you “haven’t” done (apparently) and you’ll have to redo them unfortunatly
This is why i’m waiting until the Title update to do solo legendary 
Ouch. This happened to my brother, who saved and quit and went back to finish that level later.
Every resumed fine until he finished watching the prologue and was like. . Why isn’t the achievement popping?
That level didn’t register as completed for whatever reason.
I beat it solo on legendary, but didn’t have that problem, but I can definitely see how it is annoying.
So, I have to do Mignight again!?!?!?! This is ridiculous!
Can anyone help me fix this -Yoink-?
I’ve beat this campaign three times legendary without a problem. Check which levels haven’t been completed on legendary then look at which section needs to be done. If that doesn’t work then try watching the prologue/epilogue again.
make sure your connected to xbox live when you finish the campaign
Just happened to me as well, with Mission 3 (Which is a -Yoink- on Legendary, might I add).
I imagine it relates to you saving/exiting the campaign mission, and then continuing it later, cause I did do that. Or an Xbox Problem