Ahh so thats the navmesh and it goes to a point. Making trouble…
I have seen some information about this now.
1 - If the stairs are too steep…but yours looks OK
2 - Navmesh changes as you move or scale things
3 - The collision on stairs could be causing issues, a step shaped collision gives problems, a ramp shape works better, either modifying collision model or using an invisible block to make a slope
4 - If there is a small gap between the collisions of the floor at the top or bottom of the stairs and the stairs.