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That's what I have found. If I edit right away, it does not get marked as edited. If I edit it some time later, it does get labeled. (I have a tendency to notice typos after I post the message, so I've seen this happen quite a few times.)

I can't think of any setting you may be missing. You can try clicking "mark read" on your alerts list. Normally, when you miss one alert from a given thread, the system thinks you do not need any more alerts from that thread until you go view the particular post the previous alert went with...