I think the problem here is the queueing of attacks.
If you spam attacks, like most of us do, you queue an attack during the animation of the prevous attack. I’m not sure but with some really fast quick attacks it might even be possible to queueu another attack.
Try it out, just spam quick moves and then stop. you’ll see your pokemon will do at least one more attack after you stopped.
Since you’ve then queued an attack, if your energy bar fills up the game first need to empty the queue with a normal attack and then can do the charge move.
The combination of fast quick moves and low energy charge move then might give what is described here.