Steps to reproduce:
1. Open attached project "something_2D.zip"
2. Open "test" scene
3. Observe Scene/Game view (Alternatively zoom in and out)

Expected result: black borders do not appear in the middle of the tile since borders are perfectly aligned with edit handles in Sprite Editor
Actual result: black borders randomly appear in the middle of the tile

You can also select "water" in Project View, open Sprite Editor and observe that edit handles are perfectly aligned with the borders

Note: borders also randomly change when zooming in and out (watch attached gif)

Reproduced with 5.6.0b3 (new feature)