Savage Stomp’s cost not is reducing for dinosaurs… I had a Verdant Sun’s Avatar in play, and it still cost 8 mana.
(I fetched the Savage Stomp with Karn’s first ability, if that makes any difference)
Just drew Savage Stomp on a different game and it’s cost was indeed reduced to 4…so the problem seems to be with fetching the card rather than drawing it.