Thank you so much, this makes me feel better. I'm going to mail the paper AR-11 shortly, with tracking. I hope this one sticks. I also have the email they sent me after I submitted the form online, so if this does come up when I try to naturalize I can show that I did actually file it well within the 10 days.
If this is a common issue, I wonder how many people unknowingly still have their old addresses on file with USCIS, assuming it's been updated because they filed the AR-11 online. I only called to be extra cautious and was expecting to hear they had my new address. It was such a shock.