Enhanced Pyrotechnics works exactly like the tooltip says it does. If you don't crit, you get a stack. When you do crit, the buff is removed. Unlike Wax and Wane, it doesn't say it only affects the next Fireball.
Well, it's the mechanic the tooltip says it is supposed to have. Now, why they would ever consider that a sensible idea i have no clue. Both because it is way to weak to be usefull, and because it is completely silly to have a stackable buff that cannot possibly be stacked. Thus my suggestion that it only makes sense if it can stack off DoT ticks.