Getting hit by other player and not even in pvp or duel

Hi.
I've just got a strange situation during boss hunt and I met this guy in the desert several times.
Everything should be ok if he's not trying to hit me and the thing there... he's be able to hit me without activating pvp mode or duel. Though, not all his attacks landed but I can notice through his debuffs and damages.
I was not expected this earlier but today I meet this guy again... so I decide to capture video.

My question here is: Can we really do this ? Or it is an unforeseen bug ?
I don't think this is the thread to report player, but if it is really the case I'll provide more detail in inbox chat.

Note: I'm playing on Thai Client so don't mind the Thai localize UI