Wierd...I did that quest with a Spy, he made a poorly-made fireproof potion, the king drank it, tested it out in a fireplace, got burnt, went back to my spy to complain, my spy made the 'Real' fire-proof potion, and gave it to my King. Now....since this is a legondary quest, there is always some trick to it. For my quest involving the King and Spy the quest never tasks me with drinking the real fire-proofing potion. I was standing outside that cave...and thought "wait a minute...dont I have to drink that?!" so I did and beat the quest. So I wonder if there was some interaction you were suppose to do that the game purposely left out. I don't know what it could be...but that is my guess. Was there a choice to test the shield one way or another, and maybe you picked the wrong one?
FYI...the poor-fireproof potion needs a frog, the Real one needs a leech. Save yourself some time and buy them both when you go to the villiage shopppe.
