Reproduction steps:

1. Open "tilebugtest.zip" project
2. Create a "Tilemap"
3. Duplicate the "Tilemap" inside "Grid"
4. Take a Sprite from the Palette and paint somewhere
5. Delete the "Tilemap" where you painted
6. "Ctrl + Z" Undo
7. Try to Erase the painted blocks

Expected Result: The blocks get erased
Actual Result: The blocks cannot be erased or overwritten

Reproduced with: 2018.1.0a3, 2017.3.0b8, 2017.2.0p1
Did not reproduce on: 2017.1 (Tiilemap was only implemented from 2017.2)

Note:
Workaround is to restart the Editor
Or make sure that only one TileMap exists