The reference becoming null is normal: com.unity.xr.arfoundation/Runtime/ARFoundation/ARTrackedImageManager.cs at master · needle-mirror/com.unity.xr.arfoundation · GitHub
Failure on device is likely due to insufficient image quality.
On ARCore, you can check image quality with the arcoreimg tool: The arcoreimg tool | ARCore | Google for Developers
On ARKit, you can check image quality settings in Xcode, although Apple still hasn’t restored the video that they deleted which explains how to do it. ARkit Image Tracking quality...