In Monster Hunter World does Tzitzi-Ya-Ku's stun attack have a range? I've tried looking away and I've even been behind objects yet it always stuns me so is it just an area-of-effect attack that cannot be blocked? If so, what is the area-of-effect range?
The flash-attack can be blocked by any weapon with a shield or the Greatsword and often it's not a bad idea to do so.
And yes the attack does have a range. It will hit in a cone in front of the monster and stun every player/monster in it's range. Note that in this state It's almost impossible to outrun the range if you simply run away from the monster.
Breaking The monster's head will cause the range to be reduced. If the head is broken it's possible to get out of range by running away, however this is not the optimal way to deal with this ability.
You will find the most success if you try to get behind the monster as soon as he starts his attack (It's somewhat telegraphed). This is why it's a good idea to stay close to the monster all the time.
Tzitzi-Ya-Ku will first do two preparatory, small flashes, and then the big one which can actually stun hunters. During those first two, the area of effect of the flash attack can be seen on the ground as a luminous patch.
Turning away or hiding behind objects won't work if you're standing in it, your only options are to block if your weapon allows you to, evade at the right moment to gain invincibility, or walk out of the area of effect altogether.