I think the same as you, that probably the stone transfers too much heat too quickly. Lowering the heat could resolve your problem, but where's the fun in that?
This is apparently a problem with the new stone, yet the new stone is technically the same as the old stone. Hm, something must give is what I'm thinking.
You can try cooling down the floor a bit just before you launch, just to see if it helps. I use vermiculite for this purpose, after a tip from Lilla Napoli. It works really well. I've heard other creative methods mentioned too, like pots of cold water, but those a bit risky perhaps.
(Like your clear and informative sketches by the way)