Thanks! My I-765 was filed on paper, fwiw, and neither I nor my partner have ever filed anything online with USCIS, so that theory's holding water for now at least.
I'm doing a fairly good job of staying calm -- after all waiting for a bio. letter is more-or-less where I would have expected to be at this point (NOA dated 6/23, received 6/28). It wouldn't be unrealistic to be a little further on by now, with a following wind, and it gets tougher as you see more and more people who submitted around the same time or after you getting much further ahead, but I'm staying positive. Just really don't want to be sitting here on October the 1st wondering if there was something I didn't do or didn't think to check. I'm very grateful for this thread (and especially the mods) for the fact that I have at least some peace of mind in that respect.