How it used to work:
Open a box that gives 3 items. Sometimes you would get a 4th 'bonus' card that was always a power kit.
How it currently works:
Open a box that gives 1 item. Item can be replaced with a 'bonus' card that is a power kit.
I must assume this is a bug because of how it used to work, and also because the word 'bonus' is still present. The word 'bonus' implies something extra or additional.
For example, the new Big Boy mission only gives 1 item. Instead of a 'bonus' making it 1 item plus a bonus kit, all you get is 1 kit.