How to reproduce:
1. Open the attached Unity project (tilemaps.zip)
2. Open the "test" scene
3. In the Tile Palette window, click on the "tile2" Tile
4. Move the cursor to the bottom right corner of the Scene View
5. Observe as the "tile1" Tiles overlap the "tile2" Tiles

Expected result: The Tiles retain their layering order
Actual result: The Tiles change their layering order

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.