All creatures take the same [base] damage
5th Edition D&D doesn't stipulate that there's an "order" to which creatures get hit first by Area-of-Effect effects, and the effect of the Piercing Arrow Shot is to turn an arrow into an area-of-effect.
Piercing Arrow. You use transmutation magic to give your arrow an ethereal quality. When you use this option, you don't make an attack roll for the attack. Instead, the arrow shoots forward in a line, which is 1 foot wide and 30 feet long, before disappearing. The arrow passes harmlessly through objects, ignoring cover. Each creature in that line must make a Dexterity saving throw. On a failed save, a creature takes damage as if it were hit by the arrow, plus an extra 1d6 piercing damage. On a successful save, a target takes half as much damage.
The piercing damage increases to 2d6 when you reach 18th level in this class.
—Arcane Archer, Xanathar's Guide to Everything, pg. 29
So while some DMs might interpret this to logically mean "it hits the first target in the line, then the creature behind them, then the creature behind them, ...", the rules don't expressly say that this is how it is meant to work, and in most other terms (things like Saving Throws, etc.) the damage is occurring simultaneously against all targets.
So by my reading of the rules [and their absence of any clarifying information in relation to this question], if the damage occurs simultaneously for all creatures, then that also means the arrow must "hit" all creatures simultaneously as well. So because no one creature is hit before any other, all of them must take the bonus damage associated with the arrow, if it applies to them (like in the Arrow of Slaying example).