I have completed Nightfall on Legendary 3 times, and each time, it has failed to register as completed on the main menu and preventing me from getting the Gods Must Be Strong achievement. I have encountered this bug once before, but it resolved itself on the second run through. However, now it keeps on happening and there is nothing that I know of to circumvent the issue. Am I missing something obvious, or this there a workaround? I’d hate to run through it again for it to not register again.
Photo proof of the 2nd and 3rd times: Imgur: The magic of the Internet