That's... fairly standard for MMR systems?
I've seen some streamers gain MMR for 6th place, I've seen others lose MMR for second place. All just depends on the specific lobby, and what your personal rating is relative to everybody else.
Like... think about Chess ELO - if Magnus Carlsen got first place in some local pub tournament (that also happened to matter for worldwide Chess ELO), it likely wouldn't impact his ELO score because he's at such a high level compared to everybody else that it wouldn't make sense for him to gain any points.
The battlegrounds MMR system *is* the equivalent of ELO, so if you lose in an 8k lobby as a 2k player, it isn't going to really matter, but losing in a 2k lobby as a 12k player is *devastating*