As someone mentioned before: It’s not a bug, but an intentional change and tbh I welcome it on an unmoderated battlenet.
Before this change, there were groups of players who heavily abused AT vs RT in 4v4 to boost their stats/MMR.
It worked like this:
They purchased and created a shared “loser-account” with 0 wins and almost 0 MMR and then they joined 4v4 AT vs RT with 3 decent players + the loser account (who’s a decent player too) vs 4 random players.
Normally high MMR players wouldn’t gain any MMR anymore at a certain point by winning games, but with the 1 loser account in their team, they draw down the calculated average MMR of their team immensely. Hence, they kept farming MMR up to 9999-10k MMR.
Of course that’s pathetic af, but it killed many games for a lot of people.
I prefer how it is now, since Battlenet has no moderation at all to sanction such abuses.