After accessing BackgroundPlane.GetComponent<MeshRenderer>().material.mainTexture;
(even just accessing it)
background plane will no longer update and stay on the last captured frame.
If you do not change modes, or you do not access the background texture everything works fine.
I need some dedicated support here, this issue was not present on previous version of vuforia (using 8.1.7 and unity 2018.3.12)
Edit: even just doing this will cause the issue
var WhatIsGoingOn = BackgroundPlane.GetComponent<MeshRenderer>().materials;