Reproduction steps;
1. Open the attached project.
2. Build it to an iOS device
- Allow it to use the camera if asked.
4. Point it to the attached picture "p1.tex.jpg".
- Make sure a cube spawns on the picture.
5. Check the Xcode Console
- It will print "Trackable p1 found" (Not the last line).
6. Click the Sleep button on a device while tracking the picture.
7. Check the Xcode Console.

Actual behavior:
- "Trackable p1 lost"
- "Trackable p1 found"

Expected behavior:
- "Trackable p1 lost"

Notes:
- Screenshot with the Xcode log attached.
- Xcode project may fail and ask to rename a function. The project can be built by renaming that function to a proper name.
- The project doesn't work with 2017.4.0f1 and 2019.1 versions of Unity.

Reproduced with:
- 2017.4.6f1, 2017.4.15f1, 2018.2.16f1, 2018.3.0b10
- iPhone X iOS 12.1, iPod Touch 5 iOS 9.3.5

Not reproducible with:
- Android