If you use Level Instances or Sublevels or other multi-level mechanisms, it seems you need to have a NavMeshBoundsVolume inside your “root” level - even if it’s completely empty, and even if the navmesh is being built correctly when looking at P path visualization mode.
Without a nav mesh bounds in the “root” level, it seems the navigation system is unable to find the navigation data, even if it appears to have been built and the recast navmesh actor exists within the root level.