We can produce a crash consistently with the Wikitude SDK 6.0 library Instant Tracking Sample for Unity on iOS 9.x on an iPad Air Gen 1. Upgrading to 10.x resolves the issue (although over 15% of users are still on 9.x).
The procedure is as follows:
1. run sample app
2. select Instant scene
3. initialize tracking
4. lose tracking
5. turn device screen off (using power button) and back on (swipe)
Please let us know if this issue be resolved in a future update.