Reproduction steps:
1. Open the user attached project "TestGrabPass.zip"
2. Switch platform to Android
3. In the Project window under "Assets/Resources/Scenes" open "SampleScene.unity"
4. Make sure that in the Player Settings under Resolution and Presentation Blit Type is set to Auto
5. Build for Android

Expected result: The Grabpass works when the Blit type is set to auto (See notes)
Actual result: The Grabpass does not work when the Blit type is set to auto

Reproduces on: 2018.4.26f1, 2019.4.9f1, 2020.1.4f1, 2020.2.0a21

Reproduced with these devices:
OpenGLES 2 and OpenGLES 3:
N/A, Samsung Galaxy S9 (SM-G960F), Android 8.0.0, CPU: Exynos 9 Series 9810, GPU: Mali-G72
VLNQA00313, Huawei Y6 Ⅱ Compact (HUAWEI LYO-L01), Android 5.1, CPU: MediaTek MT6735, GPU: Mali-T720

Does not reproduce with these devices:
Vulkan:
N/A, Samsung Galaxy S9 (SM-G960F), Android 8.0.0, CPU: Exynos 9 Series 9810, GPU: Mali-G72

Notes:
-Issue does not reproduce if Blit Type is set to Always
-Enter Play Mode in the Editor to see the expected results