To reproduce:
1. Open the attached project;
2. Open "Scene" scene;
3. Create a new "Post-Processing Profile" (Right-click on the Project Window > Create > Post-Processing Profile);
4. Drag and drop the "New Post-Processing Profile" to Main Camera Post Processing Behaviour Script;
5. Enter Play mode;
6. Exit Play mode;
7. Click on the "New Post-Processing Profile", check and uncheck Antialiasing in the Inspector;
8. Enter Play mode.

Note: This is only reproducible with Post Processing Stack v1. Does not reproduce on Multi Pass Stereo Rendering Method.

Expected result: Game View is not split after turning on and off Antialiasing with Single Pass Stereo Rendering Method.
Actual result: Game View is split after turning on and off Antialiasing with Single Pass Stereo Rendering Method.

Reproduced on versions: 5.6.3p4, 5.6.4f1, 2017.1.2p1, 2017.2.0f3, 2017.3.0b4.
Not reproduced on versions: 5.6.3p3.
Regression introduced on: 5.6.3p4.

Fixed on Post Processing Stack v2.