Rank up bug??

I won my placements 5/5, 5/3, and 5/1. My rank was bronze 5 after placements. After both times of reaching 5 wins I stayed bronze 5. I was playing with my friend who was gold, so the lobbies were silver. After our 5/1 his rank went up from gold 5 to gold 4.

What’s going on? Is this a bug, how did I stay bronze 5??