When a raw NavModifierComponent gets added to a character it cuts a hole in the navmesh (as expected) but doesn't update the location with character's movement. And it should.
1. Open any project containing navmesh
2. Make sure navmesh is set up for runtime navmesh generation in Project Properties (4.7: RecastNavMesh.bRebuildAtRuntime 4.8: RecastNavMesh.RuntimeGeneration)
3. Open player pawn's blueprint
4. Add Nav Modifier component
5. Compile everything and start PIE
6. Show navmesh by typing show navigation (if it doesn't show anything leave PIE and enable navigation drawing in editor mode first)
7. Move player around. There'll be a hole in the navmesh where player started, but the hole doesn't follow the player. And it should.
Head over to the existing Questions & Answers thread and let us know what's up.