Usung Model Target Funciton, OnTrackingLost is not called, in Hololens.
So, gameobjects in children tree is not disappeared.
This issue is not occurred in Unity Pre-view mode using PC-CAM.
My Vuforia version is 7.1.34.
Unity version is 2018.1.0f2.
Please review this issue.
It's because with the Hololens you are doing extended tracking, so the tracking is never lost.
Pierre