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.
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.
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.