Hello Buyaka,
I’ve run through your repro steps and experienced the same issue. However, the issue resolves itself upon restarting the editor and moving the object to ensure it is in contact with the Nav Mesh.
Below is an image of what I’m seeing after my restart of the editor (I’m using Yellow for my Nav Area color).
Let me know if you’re still experiencing this issue after a restart. Here is the bug that this issue is related to: Unreal Engine Issues and Bug Tracker (UE-30174)
Have a great day