Though I've never done BIAB, I'm not certain, but I think I've detected this in a couple of my pale lagers lately. I think it may have been a milling issue, since for a couple of batches I was getting more fine boak particles than I'd been accustomed to. Might just have been down to a batch of malt that was susceptible to excessive husk damage under standard milling procedures. At any rate, I'm going to pay a lot more attention to milling speed, and go back to extra measures against HSA, at least all that my mere human-scale brewhouse will admit. (Conventional 2 vessel setup, direct fired mash kettle and separate lauter tun.)