I've experimented with your maps, and here's a little summary:
1. NT maps with DEM, map series 24 really disable the area features of other maps.
2. OF maps with DEM, map series 24 interact with other maps correctly.
3. Both OF and NT maps with raster, MS24 work correctly with other maps.
4. Both OF and NT maps with raster, MS36 disable other maps' area features (independently of the map priorities).
All the results for MS24 can possible be affected by the map priority. I have only tested MS24 maps with the priority set to 24 (MPC's default value).
5. The levels of MS36 maps are strictly bound to specific zooms in GPS.
6. The levels to zooms binding for MS24 maps depends on the level of details in the map display settings.
The most confusing is the difference between OF and NT maps with DEM. This is the only case I'm aware of, when the map type changes the way the maps are combined.