To reproduce:
1. Open user's project "Minimal Project - Sprite Mask bug"
2. Move the mesh object in Z axis in such way it would be placed between the Sprite and Sprite Mask
3. In Scene view, observe Mesh object's material

Expected: Sprite Renderer's material is not projected on object behind it
Actual: Sprite Renderer's material is projected on object behind it when having negative Sprite Mask (check .gif attached)

Reproduced in: 2017.1.3f1, 2017.2.1p1, 2017.3.0p1, 2018.1.0b1

Note: to simulate these conditions you need Sprite, Sprite's mask behind it (negative) and a 3d object between the Sprite and Sprite Mask