as I mentioned in my last post there is not general issue with the cloud functionality and it should work so far. So what I see on your video is not really a thing what could be used to identify an issue. So means there is no clear what is the reason for this behavior.
I think , you need to do some checks to find more details when the issue occurs.
So , it is not clear if this is an issue which is device specific - here to be sure you can try to test it in unity play mode to see if this effect is there and if works you can test later on different type of devices if there a different behavior could be observed.
You can use the same picture in an Image Target from file to see if this will work when you use a image from file
A second point is to check the size of your cloud database and what is the relative ratio to the size of your augmentation. Here you have the pay attention that the width of the cloud image target should be near to the real size of the picture. The second point is that then the ration to the size of the augmentation should be in the same range so that it has size which will be visible on the display
You can also change the ARCore option in the Vuforia Configuration (Shift+Contrl+V)
There you can check if there is difference between using of don't_use and optional option.
I hope that the motioned points will make it more clear to understand the root cause of this issue
Vuforia Engine Support