I have experienced the following problem two times so far while trying to complete my ranked placement matches.
The game progresses and ends normally, but after the final round when the chatbox says "Submitting results..." I get a black screen saying "Server shutdown. You have been kicked."
And when I go back to main menu, it doesn't show that the match happened. So instead of showing that I have completed 6/10 placement matches, it shows 4/10 (because this has happened to me twice).
My internet connection is stable and doesn't disconnect during or after playing.