"Navmesh needs to be rebuilt" after update to 4.8

Just ran into this same message with a different resolution - in my case, I had my nav mesh bounds volume in a sublevel, which would make the mesh look like it built right and would display in the editor, but wouldn’t work in PIE until I moved the bounds to the parent level.

Hope that helps someone else, or helps me when I inevitably do the same thing a month from now and find this again.

I had this problem then I searched for all navmeshes in world outliner and saw some of them were not in parent level so i moved them all in parent level then I removed the RecastNavMesh-Default actor that was below all navmeshes in world outliner and built again.