We have an app that enables and disables trackers depending on needs/context. We have been experiencing an issue on iOS where occasionally, but often, after returning to the app from background that the smart terrain system is broken. A shortened version of our logs look:

[ARManager] App Unpaused
INFO/AR(11251) 2020-05-22 14:12:00: Vuforia resumed.
Start Vuforia Engine
[VF_PlatformController] OnVuforiaPaused: False
[ARManager] UpdateTrackers: imageTracker: False deviceTracker: True surfaceTracker: True faceTracker: False
[VF_Tracker] TryEnableTracker on: ImageTracker : False
[VF_Tracker] Successfully stopped tracker: Vuforia.ObjectTracker
[VF_Tracker] TryEnableTracker on: DeviceTracker : True
Starting Positional Device Tracker.
[VF_Tracker] Successfully started tracker: Vuforia.PositionalDeviceTrackerImpl
[VF_Tracker] TryEnableTracker on: SufaceTracker : True
Starting Smart Terrain
[VF_Tracker] Successfully started tracker: Vuforia.SmartTerrainImpl
[VF_SurfaceTracker] Checking for plane hit
ERROR/AR(11251) 2020-05-22 14:12:04: Failed to do hit test. DeviceTracker seems to not have been started.
[VF_SurfaceTracker] CheckForPlaneHit FAIL at screenPosition: (0.5, 0.5, 0.5)

At that point, after getting the "Failed to do hit test. DeviceTracker seems to not have been started", I am unable to get the smart terrain tracker to work even after trying to restart it and the positional device tracker. Only restarting the app will resolve the issue.