On android, there is no way to detect when an image is not tracked by the camera anymore. Once the image is tracked, the tracking status of the image is always "tracking". According to GoogleARCore doc, the status of a non-traking image switch to "pause", but there is no such tracking state in the EARTracking state enum.
There is always differences beetween IOS and Android even if the code is same. So how to know to destroy objects when augmented images are not tracked anymore by the camera for both IOS and Android?
'Get Tracking State' enum on an ARTrackedImage Object (Image attached)
Deploy to Android
Result: enum updates on iOS but not Android
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-81971 in the post.