To reproduce:
1. Open a project with ARKit plugin (I used the one from here: https://blogs.unity3d.com/2018/06/14/whats-new-in-unity-arkit-plugin-for-arkit-2/);
2. Convert the project to LWRP;
3. Switch platform to iOS;
4. Build and run any scene on iOS devices.

Expected result: Camera feed is seen.
Actual result: Camera (of the device) feed is not seen when using LWRP.

Reproduced with: 2018.3.7f1, 2019.1.0b5, 2019.2.0a6

Devices under testing:
Reproduced with:
VLNQA00161 iPad Mini 4, SoC: A8, iOS: 11.4.1
VLNQA00189 iPhone SE, SoC: A9, iOS: 11.4.1
VLNQA00157 iPhone X, SoC: A11, iOS: 12.0.1
000314 iPad Pro 11’’ SoC: A12X, iOS: 12.1
VLNQA00182 iPhone 7 Plus, SoC: A10, iOS: 12.1.3