Just because you have a successful FD does not mean you have cleared all your hate. Multiple successful FDs, dying, or zoning will do this. However if you have a huge train of mobs, the probability of losing all aggro with a few FDs is slim (unless they are static mobs and go back to their spawn point, which from what I remember would clear hate if you were still FD).
I believe that is why they put in the 2 min rule on live where if you are FD for 2+ min, it clears all hate and sends you a message telling you so. Don't expect that here though
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You can test this with one mob and multiple FDs. Just get aggro, then FD... if it attacks after you get up, FD again.. repeat. Eventually it will "forget" you.