I had the same issue.
In the normal gobo selection the physical value affects the thumbnail.
With a physical value of 0 the thumbnail is always shown. If your gobowheel is indexable you set a physical value of -0.5 thru 0.5
In the shake function the physical value is to set the shake speed. But the thumbnail is also reacting to this physical value?
I think this is a small bug or feature??