The goal is to create 2 teams of the same MMR point averages, and when possible keep the lowest player from have no hope vs the highest player far as "scoring" probability goes. Normal games are lower quality in that there is no limit on premade groups so that scoring probability can be violated.
You have played a high # of games with Players which changes the match average to be higher. Because you win many of these games that adds more MMR to your account as it shows' you did not function as dead weight to a level your teammates are incapable of winning. Depending on how Riot has their Variable for expected match results you might gain Extra MMR for winning those games as the system trys to move you up, even if it was cuz of your friends pulling a majority of weight.
Example, you play high MMR players when you que with your Diamond friend. By winning multiple games you ramp up your MMR and you look like a smurf rather then a total newbie because your winning games with averages that are above your MMR
The main goal of MMR systems are to find your effective range, where your not dead weight who instantly loses games and your not undervalued where you can single handedly stomp everything on the other team.
If you duo constantly it makes your MMR represent you + friends for your expectations rather then any personal skill, and that can suck when you start playing solo and your higher raited then you can function. Nothing left to do in that situation but try your best.... and lose till your into a range that's more accurate to your skill so you don't deadweight teammates.
The next bit is in theory and not supported by any dev data, treat it as an opinion not fact, but you can try to finish and hit 30 on different game modes and hope that helps. The pre 30 experience looks to prioritize protecting newbies from smurf s so if you get sorted into a non newbie value your experience is gonna be pretty bad. I think this is accurate but its anecdotal at best so I cant in good faith assure you it would be worthwhile and that your gameplay experience is going to get better when blended into the majority of the player base normal curve because I have no proof that is what happens.