about 5 years ago - UbiMorning - Direct link
From Y4S3.3 patch notes:[rainbow6.ubisoft.com]
With Y4S3.3 the MMR Rollback system is getting a few improvements that allow us to more appropriately target and address those who exploit the integrity of the ranked system. A drastic MMR change is a sign of massive irregularity and a red flag in our system. So we will now be resetting MMR for such players.

This means that instead of undergoing a significant change in MMR, players will have to redo their placement matches to get back appropriately on the ladder. Players who have their MMR reset will also not receive the seasonal ranked charm for their highest rank prior to the reset.