The bug isn't a bug with Flash attack, but a bug with stamina. I used to be able to do the same by trying to run when my stamina was empty. I had more trouble replicating it than you did, though.
There must be a bug that allows the stamina to drop below zero, and then reset because your stamina is invalid.
Thats exactly what I was thinking. When I try to do a second flash attack at say, 2 points of stamina, after I teleport and kill the enemy the bug resets my stamina. Incidentally, I can sometimes cause my stamina to reset after one flash attack if I spin attack quickly enough after I warp to the new location.
