The card shadowform on the playhearthstone website says it still upgrades to mindshatter when used a second time. Yet, on the client it does not upgrade the hero power. The hero power refreshes but it stays at 2 damage instead of going to three. If this is a change that I missed I apologize, just comparing the information from the play hearthstone website to in game.
Not a bug. The Core version of Shadowform doesn’t upgrade your Hero Power a second time. It does, however, reset it. You could Hero Power > Shadowform > Hero Power > Shadowform > Hero Power for a total of 2 healing and 4 damage for 10 mana. 
If you’re playing Wild, ensure you’re using the Legacy version of Shadowform. (In Classic, you can only use the old upgrade-y version)
Thanks for the feedback! I will keep that in mind. 