Positional Ranks

Not sure how the system is supposed to work. So I started the season playing mid as it's my main, and I started off in Plat 4, going 7-1 to get Plat 1 and some LP. After a few games to get to diamond, I got filled into ADC and played the game out. I lost but for my first placement match as ADC I'm starting in Plat 1? Can you explain why this is? Why did I have to start out at Plat 4 for my main role but Plat 1 after one game as a loss in a role I don't even play? Should I have got placed on a off-role first before playing my main? Because I'm having to grind this out from way farther back instead of just getting placed in Plat 1 like I have for the past 5 seasons after my placement matches.