Bring me the heart of Snow White.
Right. I've unlocked the area of the Exalted Plains where the dragon is. (gotten zapped by it too). Killed a lot of creatures, gotten a regular wyvern heart... Finally found the Snowy Wyvern, killed ...
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I had the same problem - last night killed Snowy Wyvern, lost the quest marker, couldn't find another snowy wyvern... etc. This morning I went back to Skyhold to do some stuff, then went back to Exalted Plains to search for the Snowy Wyvern loot... and I found another Snowy Wyvern :D and it dropped loot this time!
So hopefully if you just leave the area and then go back, you'll find one and will be able to get the heart.
