Years ago now, the Devs said they relaxed the MM rules when it comes to new accounts. If the MM can’t find enough low level accounts to match in a reasonable time frame, as you said, they will just be matched with veteran accounts.
This was because there weren’t enough new players to support the new player queue, and many new players were waiting almost an hour to find a game in QM. This fix can cause issues, but I really can’t think of a more suitable compromise.