[2019] Why do i lose mmr when a ♥♥♥♥ING random with cheats gets banned in my team?

Thanks for your feedback. As we mentioned in our MMR Roll Back[rainbow6.ubisoft.com] blog post:

In test-cases where we roll backed MMR only from when a team with a cheater wins a match (only victories, no defeats), we found that it severely imbalanced MMR equity as the MMR they lost from losing remained, but MMR they gained from winning was rolled back. This led to exaggerated MMR compensation spikes (as seen on the chart below), and consequently tanked the MMR of players that played with a cheater (intentionally or not), specifically in the low Copper ranks. This one-sided removal of matches is not an ideal system, and we have concluded that, win or lose, rolling back MMR from all matches that have a confirmed cheater present is a more effective solution.

While I understand that it may seem unfair, the reason for the system is to completely nullify any results (positive or negative) the banned player may have had on matches. It's essentially like those matches never happened.