After playing Campaign on Apocalypse I decided to try Boss Rush on Nightmare. After dying I went back to Campaign, but I noticed that I was killing enemies as fast as on Nightmare still. Damage I was taking was correct, but enemies kept their health pool from Boos Rush on Nightmare.
#Health pool of enemies transfer from Boss Rush to Campaign
modest granite
vital garden
And I first noticed this very early after the DLC, so before last patch even. My Apocalypse adventure saw a drastic dip in enemy HP after going through Boss Rush. However, it doesn't seem 100% consistent that it happens so I'm not sure on the exact trigger method. I have since experienced it once, since the newest patch. Am also on PS5 so, presumably affects all platforms.