[2021] Isn't immunity supposed to take precedent over 'cant be prevented'?

Edit: Ignore my first response and sorry for the confusion. Looks like this might actually be a bug. We are looking into it further now.

These DOTs are special debuffs, so B1's normal immunity can't prevent them. It is the same behavior as characters with buff immunity that can still gain special buffs, like Rey's damage immunity or Beskar Armor from The Armorer, for example. I hope this makes sense.