Steps To Reproduce.
1. Create a New 2D Project
2. Select "Multiple" Mode property in Sprite Mode > Apply.
3. Click on Sprite Editor and Slice into multiple Sprites
4. Open TilePalette Window > Create New Palette
5. Drag and Drop the Sprite in the Tile Palette window

OR
1. Open the attached Project > Sample Scene
2. Click on Window > Tile Palette > Select Palette456

Actual Result:
Tiles are rendered in the TilePalette window.

Environment:
Occurring on both Windows 10, Windows 7 and Mac OSX 10.13.

Reproducible on: 2018.2.0a4, 2018.2.0a3
Works Fine On: 2018.2.0a2, 2018.2.0a1, 2018.1.0b11, 2017.3.0f3