David I feel your pain.
My T4 did the slab shimmy dance also I was lucky no damage.
I had it it slabbed in a Mylar sheet in the holder to fix the issue. Otherwise every time I looked at the card and tilted upright Mr. Weaver would begin his migration south for the winter.
I agree not the end of the world but also not an non issue either. It's a problem they have known about for sometime. I look at it this way if Branson can slap people into sub orbit I would think a grading company could figure out a decent holder for the services and fees being charged....
John