Gets demoted by winning in 5v5 Ranked Flex?

After playing my 10 games of placement I got the info that I will be Silver IV the next season. During one of recent match I win a 5v5 Ranked Flex with Soraka and gets the info that I will be Bronze I the next season. How does this shit work?! So I get demoted by winning? Riot please tell me, how does this thing work?
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