Matchmaking Bug?

I just finished my Ranked 5 Promos and I have a question for riot for a little clarity. We had just got rofl-stomped to end us at 3 Wins and 2 Losses. We were then promptly placed in Bronze 4. After the fact we background checked the Sion top laner that we faced that put us in the smack down like he was Randy Savage throwing around Middle Schoolers and we found out he is currently DIAMOND 5. Last season he was MASTER TIER. RIOT PLEASE. If this is actually ok, just tell me why ??