Dynamic Modifiers Only does not generate additional navmesh at runtime. It will generate a navmesh in the editor and then can apply modifiers to the navmesh at runtime. Dynamic navmesh generation is the only mode that will fully create navmesh at runtime. If the issue you were seeing were lots of rebuilds happening while generating with PCG, you can lock the navmesh until PCG has finished, and once it is finished, release the lock and rebuild the navmesh in the dirty area. You can also change the amount of time spent in building navmesh each frame by adjusting values in FNavRegenTimeSliceManager.
-James
[Attachment Removed]