Nope, I ran off fairly slowly, AND was extra careful during vorlaufing, having considered this might be a cause. As I watched the runoff, it did seem like there were lots of visible particulates moving through the tubing. Not sure if that was the broken-down FB, or pulverized base malt like my other guess.
Since no one's jumping on any other obvious culprit, I'll write-this off as a one-off and watch closely the next time my FB % is so high.
Thanks all!