My son and I did the ascendant challenge twice. First time, the tincture might have worn off when he opened the chest - but he did open the chest. So we did it again the next day, and he carried the sword all the way to the chest, opened it, and even exited the portal - still no credit.
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One more thing - he even got the triumph for doing an ascendant challenge, but not sure if it was on the 1st or 2nd completion.