I doubt it's a bug. On Easy/Normal each valley ship drops 3 covies rather than the mere 1 you get on Heroic. Thus, as we go from Easy/Normal to Heroic to Legendary, there are fewer and fewer covies dropped. To me that suggests design. Maybe to do with balancing things out in regard to the amount of covies you get in the survivor areas? That is, balancing to try and ensure that the Xbox won't end up straining too much.

By the way (unrelated), you can bounce onto that small plateau. Footage coming later.