How bad is MMR system.

Created new account, bought the Battle Pass. I played the 10 calibration matches: 2.3k MMR. Before going any further I played the Battle Pass matches: 2.7k MMR. And when I was calibrating, game puts me in a 3k lobby. I lose my first game (very close game) and throws me down to 2.7k. There I stomp two wins, I lose another one, and throws me down to 2.5 etc.... How the fuck does this shit works?