I’m not overly concerned if the anime simply diverges from the LN a bit, but it at least needs to make sense by itself.

The problem is that you need intimate knowledge and deep understanding of the source material to make an adaptation into a different media. Whoever was in charge for this adaptation had neither. I would be surprised if they did more than quickly skimming the pages at best. They skimmed the scene in the replenishment hall and the word feystone might have stood out so they put it in, without understanding why the feystone would have been used there.