How to reproduce:
1. Open the attached Unity project (tilemap.zip)
2. Open the "test" scene
3. In the Tile Palette window, click on the Tile that isn't a pink square
4. Move the cursor to the bottom right corner of the Scene View
5. Observe every pink square on the grid turn into the Tile selected in the Tile Palette

Expected result: The pink square Tiles appear as they appear in the Tile Palette
Actual result: The pink square Tiles merge with the Tile they were replaced with

Reproducible with: 2017.2.0b4, 2017.2.2p3, 2017.4.1f1, 2018.1.0b13, 2018.2.0b2
Could not test with: 2017.1.0a1, 2017.2.0a4, 2017.2.0b3, 2017.2.2p4, 2017.4.2f1, 2018.1.0f1

Notes:
If moving the mouse cursor to the right bottom corner of the grid does not produce the fifth step, it is recommended to zoom the Scene View out a bit more.