To reproduce:
- Open attached project and launch "TestImageEffects" scene
- Select Main Camera
- Click the "Render Cubemap" button in Inspector.
- In Project Browser, select "Target Cubemap" and observe that the cube map is rendered correctly
- Select Main Camera again
- In Inspector, enable any of the attached image effects (Blur Effect / Color Correction / Antialiasing).
- Click "Render Cubemap".
- Select "Target Cubemap" again and observe that the cubemap is not rendered correctly

Please see attached screen shot for details.