https://www.babylonjs-playground.com/#MU17PR#8
If you pan the camera up and down, you will notice a juggling/glitch effect of the reflections or refractions (not sure) jumping around when you have a "glass" material with a inside surface and outside surface.
*UPDATE*
Maybe its not so much PBR thing as a depth fighting thing? the environmental intensity is set to 0 on this and a hemiLight is turned on to make it simpler to see.
https://www.babylonjs-playground.com/#MU17PR#17