I think you might have a 100% chance to get looms on a difficulty you haven't done yet:
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The way Heirlooms work is you get a guarenteed drop for each difficulty that you have received a BoA before. So say you only have gotten a BoA on heroic, well now you can run normal and mythic and get one each run.
source.
So, great chance for us to bring alts to stuff if they haven't done that difficulty before. Perhaps we can even make an Alt Mythic run soon.