I ran into the same problem with my project, i tested it with a empty persistent level that only contains the navmesh volume and the recast actor.
The sublevel contained geometry and gameplay actors. After setting *Runtime Generation * to Dynamic on the recast actor, it worked fine.
You can also use navmesh modifiers or link proxy’s in the sublevel. If you then test your AI with simulate, you can see the navmesh being rebuild to fit the sublevel.