Why Not Make Prefabs
It's a bad solution
When you create a prefab by dragging a Grid with child Tilemaps to the Project folder, all references to Objects in the Scene are lost just like any other Prefab that you might create. This includes all TPT tiles. If you were to open the Prefab, the locations where TPT tiles had been placed would be replaced with pink or other oddly colored tiles.
TilePlus Won't Let You
If you mouse-drag a Tilemap or any GameObject that has Tilemaps as child GameObjects into a project folder to create a prefab, and there are any TPT tiles, the system will warn you in the console and will unlink the items you dragged from when you created this prefab so that the scene Tilemaps won’t be corrupted. You may as well delete the prefab that you created as it isn’t useful.
You Probably Don’t Want Tilemap Prefabs
They’re not really that useful except in limited circumstances. Each time you drag in a Tilemap prefab, it instantiates an entirely new Grid with Tilemap children. This is true for any Tilemap prefab, even one created normally by dragging a Grid GameObject from a scene to a project folder.
What’s more useful is being able to load tiles in groups to existing Tilemaps. You may already do this with Tilemap block move methods and so on.
TilePlus’ TileFabs and Bundles handle all this for you and are easy to create as you'll see.
TileFabs and Bundle assets are not Prefabs!
Please note that the TileFab and Bundle assets are NOT prefabs: you can’t drag either of these into a scene.
However, a TileFab archives the original source Tilemap names and/or tags.
The TilePlus library functions for placing TileFabs in a scene expects to find these same names and/or tags to place the multiple archived Tilemaps correctly. That is, which Bundle assets referenced by a TileFab should be ‘painted’ on to which Tilemap. It can’t guess.