Well, it happened again today, but this time I think I know what's going on.
I had my same notification set up and it was working correctly. I was getting emails of other finders, but the moment I logged a find that fell within that notification, I stopped getting the emails. When I checked back on the http://www.geocaching.com/notify/ page, my Instanotify that I set up was gone.
Somehow when I am the cache finder, my notification gets deleted. Weird and annoying.
I've submitted a ticket about this.