By default terrain and objects in map and map plugin mods will not be obscured by the haze as the player moves away from them. To make the mod terrain and objects act like the SimplePlanes ones, go to Window->Rendering->Lighting Settings in Unity and scroll to the bottom of the Lighting window. Then click the "Fog" box that's under "Other Settings".

This will obscure everything in the Unity editor window, so just enable it before building/saving the mod.

Works with SP 1.10 and Unity 2019.4.