Well, I know cocoa butter is known for something similar, like Lin mentioned, even in chocolate, when it has gotten warm and then cooled again. It is harmless though. I wonder if all the other butters do that as well? I know my high-shea bars get ashy, and that it was a bit waxier when I wiped it off, than, say, my carrot bars, which have more OO, but I don't steam them, I just kinda buff them with a soft cloth, and don't worry about the nooks and crannies in the tops. I don't see it as a reason to avoid the high percent of shea though, because it's only happened to the 50% or higher bars in any great amount, and didn't cause any issues with use, beyond the first lathering. It's just cosmetic. But I know also, that for some, the cosmetic is a deal-breaker, if you have a specific look you strive for.
Looking at your recipe, it wasn't just the 30% shea, you also had mango and coconut, and babassu, and you also had buttermilk. My carrot bars, which have almost no butters, only buttermilk and oils and a tiny bit of cocoa butter, get very similar ash. And I have a recipe 35 shea and 25 mango, with OO and others, that gets no ash whatsoever.
So I don't think it's necessarily the shea, but a combination of things. What oils its combined with, what liquid is used, how much it heats up, did it gel? Overheat? As it cured, was it in a warm room? Cool room? My high-shea bars didn't get the waxy ash stuff all over until I had moved them from the very cool hallway to the fairly warm main living room area of our little apartment, after they'd already cured a couple months with almost no ash, leading me to think that just like chocolate that's gotten warm, then cooled, there's a temperature issue at play here as well, at least during cure-time. And since it seems like high-shea bars need more cure time than lard or palm to get a really good lather, that the window in which this could affect them could be longer, even up to 16wks maybe? Maybe longer? I'd have to play around with this more and see... hmmm