Navigation on landscape makes all characters line up after a while

When you have a landscape with different slopes&angles and you make characters move towards the player, they all line-up after a while. Which makes it very weird as all characters walk towards the player in 1 line. I think it is related to the cost calculation and the angle of the landscape the player is currently at but I cannot find documentation on configuring the path finding. I found this URL Navmesh on terrain - slope angle? - World Creation - Unreal Engine Forums but it is not the same as my navmesh includes the whole landscape and you can correctly see the navigation mesh covering the landscape in the editor.

Any help is welcome and a big thanks in advance!

Greets,
Timothy