So in another similar exercise "Muitos cavalos têm morrido" is translated as "Many horses have been dying" and "Many horses have died" is not accepted. In this exercise it's the other way around: "Many people have been dying" is not accepted.
It should be accepted though, it's actually the proper translation.
This exercise has been driving me crazy!!! They have been dying, they have died... How can you tell???
Only by reading their answers. I asked if they had a rule but no reply.