Hello everyone just an update to this situation,
For players who have contacted us about gaining less MMR points after a match, which they believe is related to the new Shadow Legacy update -
After consulting with the Siege team leads about this since the update, they have informed us this is not a glitch.
After your placement matches and gaining your Rank, the game has a good idea of what skill you have and what uncertainty of winning stat you have.
The game does not reset these stats with each update, only your Rank. Rank does not effect MMR at all.
MMR gains and loss is purely based on your skill rating, performance as a team and your uncertainty stat.
With all this taken into consideration, the game is confident the rank you have been placed in is correct, so you will have a low uncertainty stat, meaning the game will reward you less MMR.
Only newer players will gain more MMR or lose more MMR because their uncertainty at winning matches stat is higher, as they have not played enough matches for their uncertainty stat to be lowered yet.
The best way to gain more MMR at this point is to beat higher skilled players, the game will reward you with more MMR as it did not believe you were statistically going to win the match.