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A day ago, I had completed all the gold medals at the three initial shrines offered for the new mastery challenges ("the Reckoning"). I had not done the final quest to to unlock the last door, or the final shrine, and instead took a break and went to Ravensthorpe to do the Oskara festival things. I stopped by Hildiran's shop and bought all the merchandise there with the currency earned from having all the gold medals done for every shrine except the very last one ("Mastery Energy" is the currency). So the Valkyrie armor was not yet in the shop (as I hadn't confronted Hildiran). I bought the two weapons with the gold medal currency - the "leaf bow" and I think the other one is called "raven sickles" or something like that. And, like normally happens when making a purchase of a unique high end weapon, the weapons disappeared from the shop as they're supposed to. Also bought all the tattoos and other items, except one tattoo I couldn't purchase yet because I needed more of the lower currency ("Battle Energy").
Afterwards, I went to the last door, finished the Hildiran quest, and then started working on the last new shrine's gold medals. After getting them, I returned to Ravensthorpe and went to Hildiran's shop. It had the Valkyrie armor now, but also the two weapons I had already purchased.
Fortunately I didn't make the mistake or re-buying the weapons, and instead used all the remaining Mastery Energy to buy the Valkyrie armor (and the last tattoo with the lower currency). Had I accidentally purchased the weapons though, I would have been permanently locked out of buying the Valkyrie armor, because the Mastery Energy is a limited, one-time acquisition thing from the shrines.(As is the lower currency "Battle Energy" from just completing the challenges at all in the shrines). The other lower items I had purchased before with Battle Energy did not reappear in her shop (tattoos and such), only the two new weapons purchased with Mastery Energy reappeared.
Maybe the bug has something to do with the timing of when I used the shop (using it after completing the first three shrines of the new mastery challenge, but before confronting her and completing the quest). Or maybe the bug is just that those weapons will reappear regardless of timing. In any case, thought the bug should be reported. Although I didn't get hurt by it since I caught myself before accidentally re-buying the weapons, there's probably someone out there who will make the mistake and then find they lack the Mastery Energy to buy all the Valkyrie armor pieces and have no way of getting new Mastery Energy.