How to avoid vegetables sticking on heat-resistant glass dish?
Butter has a fairly low smoke point, and may be burning in the oven. You could try using one of the "yellow oils" like canola (rapeseed), peanut, soybean or corn oil.

Aside from that, it's not completely unexpected for cooking vegetables to stick to the edge of a dish. It may just be normal behaviour. My apple pies will always stick slightly to the edges of the pie dish.