I have random problem with drawing navigation mesh. Is correctly build and working but not displaying in editor (preview) - sometimes. I created new project, fps blueprint based with basic ai and nav mesh, I had this error in other biggest project and started new one to check only.
When playing in new editor window with console command show Navigation - nav mesh is displayed as expected. Working also with other play modes.
When I placed Nav Mesh Bouds Volume on level, everything was ok, just after some time nav mesh is not drawing. Of course I check P and Navigation is checked in properties.
After disabling static lighting nav mesh is visible again. Before it was random enable, disable when changing nav mesh settings, but mostly not visible. With static light disabled, no problems for now.