Date & Time: 9/10/2017 9:38 am PDT
Frequency: Once
World name: Leviathan
Character name: Ari Kitae
Retainer name: n/a
NPC name: n/a
Monster name: Arbuda
Class/Level: Summoner 70
Party or solo: Party
In-game time: n/a
Area and coordinates: The Temple of the Fist, second boss arena (12x , 12y)
Housing: n/a
Steps:
1. Engage Arbuda in battle.
2. Have 3 stacks of Aetherflow.
3. Arbuda uses Killer Instinct (which parries attacks at 100% rate from all directions but one).
4. Use Fester on Arbuda from one of the directions that Arbuda is blocking.
5. Arbuda parries the attack, no damage taken to Arbuda as intended.
6. Aetherflow stack is spent. No Aethertrail Attunement is granted to the summoner.

The battlelog does not show stack use/granting for either Aetherflow or Aethertrail, but here is the log for the exact moment the attack did not work as intended. Timestamp is in PDT.

[9:38]You use Fester.
[9:38]  Parried! Arbuda takes 0 (-100%) damage.

According to patch 4.05:
"Aetherflow actions are no longer required to inflict an effect on a target to gain Aethertrail Attunement."

Therefore, the abilities are not working as intended for this boss. More testing needed to determine if this is the case for all bosses with abilities that parry 100% damage.