To reiterate:

Game is doing this not ACT.
It’s not related to blade it happens with hammer, assault rifle and elementalism from my tests and most probably can happen with every weapon. The only reason I first inspected blade users is because I was comparing myself with other blade users in terms of DPS performance.

And I believe it’s simply a bug that is related to lag in the servers. BECAUSE if it were about ANY passives or ANY items, then I would see this happen in every fight because I don’t change my build/gear. But I could only make this happen when I spammed 300 basic attacks on a combat dummy.

I can also prove my point by simply removing all passive abilities and use a generic (non-extraordinary) weapon and make it happen regardless.

EDIT: I removed every passive and equipped a non-extraordinary Fist Weapon and spammed Basic Ability (Thrash) at the dummy and this happened:

You start using Thrash.
Your Thrash hits (Normal) Anima-Sim: Nightmare for 179 physical damage.
You successfully used Thrash.
Your Thrash hits (Normal) Anima-Sim: Nightmare for 1021 physical damage.
Your Thrash hits (Normal) Anima-Sim: Nightmare for 1002 physical damage.

See the 179 damage hit? It’s just not related to any weapon/ability or passive.

Another occurrence (happened after another 100 tries):

You start using Thrash.
You successfully used Thrash.
(Critical) Your Thrash hits (Normal) Anima-Sim: Nightmare for 645 physical damage.
Your Thrash hits (Normal) Anima-Sim: Nightmare for 1043 physical damage.
(Critical) Your Thrash hits (Normal) Anima-Sim: Nightmare for 3988 physical damage.

This time the echo damage crit for 645. It clearly states it’s a “critical” in the game logs even.