I've been screwed over multiple times by the x10 button in this regard, throughout all of legendary days aswell as before when it was first implemented it was abundantly clear that while on occasion it's better to use the x10 method, more than half the time it will pay out more doing it one at a time, either I have bad rng or it's just not been implemented in a way that makes using the kobans not worth the risk of it not paying out how it should, which would be double the average of doing it one at a time, which for me is about 4-6 shards per 20 fights where as the x10 method seems to pay out an average of 2-4. I'm not saying it was an intentional blunder but it is a blunder none the less.