all 5 pieces of the Forge, but nothing happens when returning to the temple.

It happened to me too with the 5 pieces selected in the sovreign item menu. By entering the forge instead with the 5 pieces deselected the quest was completed and I won the game.

I encountered the same bug.  I tried it both with the items selected and not and it made no difference; it wouldn't acknowledge my win.

did you give all parts to hero who started quest  to turn in ? I have seen it posted that this matters.

I went back in just now and tried switching the forge pieces around to the heroes in the area and it did not solve the problem.  However, it has been a week since I did the quest.  Perhaps it is one of the heroes I didn't try (I have about a dozen). 

Even if that would fix it, it is still a problem as there is no way to track which hero started the quest and no logical reason why only they could finish it.  This should be fixed (and what would happen if I lost that hero but had all the forge pieces?)

If it's still bugged you should've just declined the last fight, when you surrender the guy gives you Praiden's Sword. 50 attack and 10 speed for one hand. Reeeally nice.