I was just getting back from some gem mining, and I think this happened when I was going through a portal just as the fast brew was running out. I now have 1 sec of fast brew remaining…and have for several min. I tested out that I still have all the benefits, fast moving speed and action speed without having to rebuff.
Not sure if this something likely to happen again, but I can see this being exploitable. I tested to see if going through more portals would drop it but so far still have it. I imagine logging out will remove it?