I will say, I had cleaned the top platter with IPA after contaminating it at one point in the restoration process. It left behind streaks (which are still there, even now that it's functioning). At one point before it began working, I had gently buffed with a microfiber cloth a small spot near the center of the platter, but I can't imagine that's what did it. The behavior of the drive had already begun to change, and it "recovering" itself may have been inevitable.
I suppose I know more or less now what's happening for the next time I try to recover a drive like this!