UI Scrollbars currently block Prefab Mode in Play Mode. This has some bad knockons, including breaking the Scene View grid lines in certain situations. If you have the main scene view set to 3D but are viewing a prefab with a Scrollbar in 2D isolation mode, then enter and exit playmode, the grid in the main scene view will stay vertical as if the scene view is still 2D.

You'll get the following warning when attempting to enter playmode in this instance:

Prefab Mode in Play Mode was blocked by the script 'Scrollbar' to prevent the script accidentally affecting Play Mode. See the documentation for [ExecuteInEditMode] and [ExecuteAlways] for info on how to make scripts compatible with Prefab Mode during Play Mode.
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

1. Install Unity
2. Open the attached project
3. Make sure the Scene view is set to 3D
3. Navigate to Assets Canvas.prefab
4. Open Canvas.prefab in isolation mode
5. Set the isolation scene view to 2D
6. Enter play mode
7. Exit play mode

Expected Result:
The user is returned to scene view and the grid should match the 3D view setting.

Actual Result:
The user is returned to scene view, which is set to 3D, but the scene view grid appears vertical as if the scene view is set to 2D. It will stay this way until the project is reopened.

Tested on Windows.

Occurs on 2019.2.0a1, 2019.1.0a14, 2019.1.0a7, 2019.1.0a5

Does not occur on 2018.3.0f2, 2019.1.0a1, 2019.1.0a3, 2019.1.0a4