failure of kiln wash
The is the classic effect when the glass sticks to the shelf. Normally, each time you go above 7;20C or so you need to renew the kiln wash. Some do it over the old if undamaged layer, most get rid of the previous layer and add new to the clean shelf.
I haven’t seen your shelf or the application of kiln wash to it, so I am guessing. I think you application may be too thin. For me I need to apply kiln wash – mixed 1:5 by volume – in several directions while the previous is still wet. (I mention this as one person has interpreted instructions to be like painting wood – letting each coat dry before the application of the next. This is not intended) I apply one coat N-S, another E-W, another NE-SW and a final SE-NW. As I use a pink tinted kiln wash, I can see if the shelf is completely covered. If not I do the same process again.
At any rate, the glass has apparently stuck to the shelf, so you need to review how you apply the separator.