I'm working on an iOS/Android app using Vuforia, and recently ran into an issue with one of our iOS devices.
The device is the iPad Pro 12.9 inch 2nd Generation, on iOS 12.1.4. You can find a video of the issue here, where, once I turn on and off the ARCamera a couple of times for a ground plane augmentation, the ARCamera stops working, and shows a black screen. I've attached the relevant code as a text file so that you can search through it, but as far as I can tell, we're enabling and disabling the VuforiaBehaviour.instance as expected(using VuforiaRuntime.Instance.enabled = true/false). It is interacting with many other systems, but the black screen only occurs when starting and stopping ground plane augmentations, though it persists in all augmentation types until the app is restarted.
Curiously, this bug is only present on this specific device, and only when starting and stopping Ground Plane Augmentations repeatedly within a short period of time. If the user exist the augmentation(turns off the VuforiaBehaviour), and waits 20+ seconds before starting a new one, the camera will properly present itself.
There are no errors thrown, and Vuforia logs that it and all it's trackers have started correctly. On our iPad Mini(9.7 inch) running 11.3.1 does not have this issue at all, running and identical build. It also works just fine on Samsung Galaxy S6, on Android 6.0.1.
Graphics Library is OpenGLES2
And the issue happens on solely on our iPad Pro 12.9 inch, Second Generation with iOS 12.1.4