Do you have screenshots or a database to share? The rtabmap's VLP16 launch
file is doing 6DOF lidar SLAM, but no occupancy grid is produced (RGBD/CreateOccupancyGrid=false). You may set RGBD/CreateOccupancyGrid=true, though more tuning about Grid parameters would be required for your case. You may try to compare with this ros1 example with husky, which I show a robot setup with VLP16: follow this example
minus RGBD camera and wheel odom guess.
If you didn't set RGBD/CreateOccupancyGrid=true, you may have started another SLAM with nav2, which is the one you are seeing creating the occupancy grid.
It is possible to set rtabmap in localization mode only (Mem/IncrementalMemory=false, and remove '-d'), however you need also a camera so that rtabmap can globally localize in the map (without camera, a workaround would be to send a initial pose topic to rtabmap).
For rtabmap and nav2 integration, you may look at those examples: https://github.com/introlab/rtabmap_ros/tree/ros2/rtabmap_demos/launch