Not trying to be a jerk,but I have 100% on this game (platinum trophy on PS4) It may be different on XBone,but probably not .You HAVE to do the co-op missions and get all the eagles and other collectibles to get your achievements,it may be worded incorrectly ,but it's not a glitch.You may not agree with this ,but unfortunately ,that's the way it is .
I seriously doubt Rebellion will comment on this ,but you never know ,it could happen .
Just be glad you don't have to buy all their DLC to get the achievements ,as there are eagles and collectibles in all of them as well .
Hook up with a friend ,play the co-ops ,get your achievements ,and have a great time doing it . ( Really fun missions imho )