MMR in 5v5 ranked solo/duo and flex problems

I'm currently Platinum IV on ranked solo/duo and Platinum III on ranked flex 5v5. I don't like how MMR in 5v5 ranked solo/duo and flex 5v5 is kind of separated. I just really don't think it is right that way... It makes me feel very uneasy and likely to get tilted because of that. In my last few ranked 5v5 flex games I played against players that are Diamond 2-3 on solo/duo and it's not just one game. Dont know is that right, but it frustrating and I dont think that is fair. It happened in a few games that our team MMR difference in solo/duo playing flex 5v5 is like 200-300...