Hey there,
I completely understand the confusion, but this is working as intended. This was also reported here
At the start of each Season, players will be demoted to a lower Division. The Division the players roll back to will depend on which division they finished the previous Season. This is working as intended.
In this scenario, it is normal for you to have received those e-mails. The first one congratulates you on your promotion, but because the season restarts, the other e-mail tells you which one is your new division because everyone gets demoted at the beginning of each season.
-Groguet