https://worldoftanks.eu/en/news/general-news/preferential-matchmaking-fixes/
According to WG they realized they were making a mistake by trying to change the pref MM tanks to regular premium tanks. So preferential MM will be kept but those tanks will be buffed to fit better the meta. (Lol FCM mobility and Type 59 buffs)
Also they are studying a new algorithm to fix the MM:
"In its current state, the matchmaker often fails to ensure proper rotation between the top/middle/bottom of the list. Getting matched at the same position for several battles in a row slows down progression