Noted Bug. I've seen this happen repeatedly usually when I'm getting attacked, since I rarely have the Bonus Damage 2. However I can definately confirm it is not working for at least the character Fifty, as my opponent attacked me with him during a match with the bonus active and I only took 5 damage. The Fifty was lvl 2 at the time, which is when he does 5 damage normally.
I do know it works for other characters Lao for example has no problems hitting me for 10. It may just be Fifty or the Bangers team in general. More testing is needed but this is definately something I've seen happen more than once.