Reproduction steps:
1. Open project in "RectMaskBug.zip"
2. Build for iOS and Deploy the application to an iOS device using Xcode
3. Inspect the phone screen

Expected result: RectMask2D masks the green image correctly, it has a white border around it (Expected.png)
Actual result: RectMask2D fails to mask the left side correctly, there is no white border on that side (Actual.png)

Reproducible with: 2019.4.10f1
Could not test with 2018.4, 2019.3.0a1, because the references get removed, 2020.1, 2020.2 because iOS 10 is not supported

Devices tested:
Reproducible with:
iPhone 5C (iOS 10.3.3)
iPad 4 (10.3.3)
Not reproducible with:
iPhone 6 12.4.8
iPhone 6S 13.3.1
iPad Pro 12.9 1st gen (iOS 13.4.1)
iPhone 8+ (iOS 12.0)
iPhone XR (iOS 13.4.1)
iPad Air 2 (iOS 11.0.3)
iPad 9.7 6th gen (iOS 12.1)
OnePlus A5000, Android 10, Adreno 540, Snapdragon 835
Samsung Galaxy Note9 (SM-N960F), Android 8.1.0, CPU: Exynos 9 Series 9810, GPU: Mali-G72
Google Pixel 2 XL (Pixel 2 XL), Android R, CPU: Snapdragon 835 MSM8998, GPU: Adreno (TM) 540
Huawei Y6 \xe2\x85\xa1 Compact (HUAWEI LYO-L01), Android 5.1, CPU: MediaTek MT6735, GPU: Mali-T720
Samsung Galaxy S20+ 5G (SM-G986B), Android 10, CPU: NOT FOUND, GPU: Mali-G77

Notes:
1. This issue seems very similar to the one in case 1228393
2. This only seems to occur in devices which support only OpenGL ES 2 and nothing else (full list of such devices can be seen here: https://developer.apple.com/library/archive/documentation/DeviceInformation/Reference/iOSDeviceCompatibility/DeviceCompatibilityMatrix/DeviceCompatibilityMatrix.html#//apple_ref/doc/uid/TP40013599-CH17-SW1)