To reproduce:

1. Download attached "ScreenToWorldPointBug-repro.zip" project and open in Unity
2. Open "Test" scene
3. Build for iOS
4. Deploy the Xcode project to the iOS device
5. When App is started, use the Home button to try to switch the App to the Background - only try to switch, not actually switch the App
6. Switch the App back, and click the screen
7. Observe that in Xcode an Exception will be thrown with a large value:

"Screen position out of view frustum (screen pos 340282346638528859811704183484516925440.000000, 340282346638528859811704183484516925440.000000, 0.000000) (Camera rect 0 0 1792 828)
UnityEngine.Camera:ScreenToWorldPoint(Vector3)"

Notes:
- This issue happens only on iOS
- This issue does not reproduce on Android (Mono and IL2CPP) and Standalone

Tested with:
- iPhone 7, iOS: 10.2 - Not Reproduced
- iPhone 8 Plus, iOS: 12.2 - Not Reproduced
- iPhone X, iOS: 13 - Reproduced
- iPhone XR, iOS: 12.0 - Reproduced

Reproduced on Unity 2017.4.0f1, 2017.4.32f1, 2018.1.9f2 and 2018.2.0a5
Not reproduced on Unity 2018.2.0a6, 2018.4.7f1, 2019.2.2f1, 2019.3.0a12 and 2020.1.0a1
Fixed on Unity 2018.2.0a6 (9e4973676e75)