Monk Not Always Proccing From Behind?

[Teiren]

Perhaps what you're confused about is not getting the bonus damage while solo. When solo, there is no way to get a rear bonus and no serious way to get a flank bonus if the target isn't performing an ability or spell or isn't stunned because they're always turning to face you before you count as being at their flank or rear. It doesn't matter what you visually see because everything has a delay in this game by design; it's not lag. Other than that, every offensive action is counted/calculated a split second after you start performing it, before it hits or visually takes effect.