I can recreate the same issue probably 100% of the time by starting a Reply to a post, going away from the Reply's tab/window in my browser for a while, then coming back and finishing the reply. I don't know for sure what the time frame is for how long the Reply window must be open to create the issue, seems something like maybe 30 minutes or so.

It's always done that BTW. I believe I checked it with both Safari and Firefox. It's probably a cookie issue. Like you said no big deal though.