Ok, so beating the last level on normal, different ending got me the normal achievement, but not the easy one. Not sure if it's because I'm offline without an update though, as the timestamps from other people indicate that they got all 3 difficulties at the same time. 3 different Dog Tags (that count towards achievement counter), but the menu says the level only has 2, lol.
Also, according to timestamps from someone with "all assignments and collectibles" achievements, he unlocked it when he got 28 collectibles. I have 15+ collectibles, and like 1/19 Dog Tags, so the math does not add up at all. Not even a little.
Anyways, last ending is up, and playing on easy...
I don't think the tags are that complicated to be honest. I think there are just unique ones for the final mission, for when "special" characters die off.
I was going to say..so, glitched for you? :/ Maybe it has something to do with how the game switched to easy for you or something that you were telling me the other day, but you loaded back into hard didn't you..