I’m currently in the Underdark league, so I can’t speak for the other leagues.
But there is a clear breakpoint in how matchmaking versus humans is performed at 3,700 trophies.
During the initial phase (before the first bot prompt) the matchmaking occurs as normal and looks to find a player within 200 trophies.
If I wait for a human opponent, it will ignore all players of lower trophy count and only match against players of higher trophy count. Even though I know there are lower ranking players who are currently matchmaking.
Once I drop below 3,700 trophies, if I decide to wait, both lower and higher rank players become accessible for matchmaking.
I believe this hidden matchmaking mechanism accounts for the “hot” and “cool” streaks that some players have reported on the forum.
This would also create a “gravity well” for keeping players near the middle of a particular league, and cause them to have about the same amount of trophies for their account maturity throughout a season.
Please post supporting or refuting experience in the Underdark League or if you too seem stuck in the middle of your League due to matchmaking “luck”.