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I don't have video proof for the same situation in the video below in pre-TU17. But it seems a single enemy can no longer contribute multiple(2 to be specific) stacks of Adrenaline Rush bonus armor(BA) and this is how the experience goes for me.
Pre-TU17 experience:
Adrenaline Rush will continuously provide bonus armor(BA) stack as long as 1 or more enemies are within range. But it is possible to gain multiple/max stacks(not more than 3) even with 1-2 enemies. They don't appear all at once but rather with a 1 second delay, unless more enemies comes in range to contribute more stacks. In the same Fieser fight shown in the video below, it is possible to gain all 3 stacks of BA when standing between the train and Fieser. While Fieser and the train contribute to 2 stacks of BA, about roughly a second later I'll gain the third stack of BA BEFORE the previous 2 stacks cooldown expire. If the train is not in range but Fieser is, he will contribute to 2 stacks of BA with the 1 second delay. As I recall it is not possible to gain 3 stacks with just Fieser as the first BA cooldown will expire and a new stack take its place.
Post-TU17 experience:
Adrenaline Rush bonus armor(BA) gaining is limited by the amount of enemy within range. Each enemy can only contribute a single stack of BA so 3 enemies are needed in order to gain maximum stacks of BA. During the Fieser fight it is no longer possible to gain all 3 stacks of BA inside the Boss room since there are no more than 2 enemies comes within range. As proven by the video I'll only gain 2 stacks of BA when between Fieser and the train. Once the train is out of range I only gain a single stack from Fieser which is different from my Pre-TU17 experience.
Reason of noticing changes:
I used to run a tank build with about 70K armor regen + 100% fire resistance inside boss room and I'll be able to survive as long as only Fieser or the train is within range in Pre-TU17. The same build can no longer survive with just Fieser as I'll need 2 stacks of BA for my armor regen to overcome the damage taken.
I'm not sure if this is considered a bug or just a stealth nerf as I'm not seeing any changes to Adrenaline Rush in the TU17 patch note.