Days Gone is a great game, a bit too much repetitive, but still OK. But sadly, like every game, it has some problems and bugs. Even bugs that stop your progress in game.
When you are in the mission Tail the Horde, you have to follow Iron Mike around the Horde. But every time you come near the Horde, they see you and the mission fails.
How to beat the mission?
What you have to do is wait. Yeah, just do that. Before going near them, just wait until they have all gone up the mountain and are not on the road any more. Now you can safely get up that hill while talking to Iron Mike and finish this objective.
Sadly, it is not expected to be fixed by the developers, so this may be your only solution about this problem.