The current SBEC MM we have now is completely different from the ARB one.
Matchmakers in ARB are influenced by the population of players playing a particular BR ranges, which determines how often you get an uptier or downtier, and if there are too many players playing a particular vehicles (e.g. Ju 288C, F-5C, F-4S, etc), it will have a negative impact on matchmakers, and the frequency of getting uptier and downtier games, which should be 50:50, will be highly skewed.
Mostly cases people bring these vehicles are just grinding so, it is almost impossible to fix it if we stuck at current matchmaker system because people spam it even if these vehicles moved to higher BR.
The details are all in the WT wiki, but basically SBEC has no random uptier and downtier like ARB, instead the BR blanket is rotated every few dozen hours.
Since some BR blankets are +/- 1.3 BR, they need to be fixed for ARB, but if this is implemented, it should fix the “BR blackhole” issue. It can always be meta for a few days for almost any vehicle, unless it is extremely overtiered, and even implementing in ARB it doesn’t require that much work, as it is just a matter of C&P from SBEC matchmakers that is already exist.