Augmented reality - UI - How do I augment an User Interface Canvas properly?

Hello, I made a canvas with some buttons (scale up button & scale down button) for my augmented 3D models.

One ImageTarget (QR1) has 3D model cars as children, the other (QR2) has a Canvas with some buttons. When both ImageTargets are detected, the cars and Canvas properly are augmented. But when I remove ImageTarget QR2, the buttons remain on screen. (please see image attachments)

I would like the Buttons to disappear when QR2 is not detected. What can I do to make this possible?

I already disabled Extended Tracking of both ImageTargets but that does not seem to help.