2 issues with competitive

First issue: I used to be Master 4, now I dropped to Diamond 1. I’ve just played in stack with my friend who is Plat 1. Why are ALL other team members Plat 3, Plat 3 and Plat 5? All remaining team members are lower in rank than my friend! Why do I have to play with Plat 5 which is almost gold while I am almost Master?

Second issue: ranking after 5 wins. I play 5 wins 3 losses and get the same rank. I just played 5-5 and got deranked!! Why?!