STAT_Navigation_CollisionTreeMemory and STAT_NavigationMemory are not accounting for DEC_MEMORY_STAT_BY OctreeSizeBytes in the navoctree destructor. It appears STAT_Navigation_CollisionTreeMemory ElementMemory is also not being decremented.
There are other potential code paths that do not seem to be accounted for too like FNavigationOctreeController::Reset().
Create a single player level with some geometry and a dynamic navmesh.
Play the level and display navigation memory stats
Issue RestartLevel Console Command.
Note nav octree stats grow each time the level is restarted.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-166263 in the post.