If you're curious to know how 'much' of your own Mapshare data is being retained on a Nav2 unit, just look in your map folder for "MapUserPatch.dat". Unfortunately, that info is wiped with every new map update For some folks, I'd recommend storing a copy of that file before a map update and restoring it afterward to see if it works. As long as TomTom doesn't fiddle the format between releases, this should restore Mapshare changes. I know that at one time, we had users 'sharing' this data between their fleets of several units in this way. It MAY also require saving and restoring the UserPatch.dat file from the root of the device as well. It's been quite a long time since we had to steer anyone in this direction so that they could copy changes from one unit to another.
Here's a thread on retaining Mapshare changes you make for your own unit:
https://www.tomtomforums.com/t24887-will-mapshares-carry-over-new-map.html