While this is not a bug, there is an unintended consequence of allowing heroic treasures to be exchanged for gold coins directly.
All you need to do now is carry the treasure outside of the no build zone, build a coffer and exchange the heroic treasure for 5000 gold coins (50 weight), dismantle the coffer and continue on with your adventuring.
Solutions:
Add a 15 minute crafting time for exchanging heroic treasures.
Add a 30 minute timer after building a coffer before the exchange treasures button works. (you could still build a small shack outside a known heroic treasure zone and constantly exchange there though)
Heroic treasures are the only treasure that cannot be exchanged for gold coins. (i dont like this one)