What happens if i don't finish my placements before the season ends?

Not 100% on this but I believe it does. I played ranked Twisted Treeline last season with some bronze friends but never finished placements. I did all 10 of my placements with the same 2 people this season. My Platinum 4 friend placed Gold 5. My Gold 5 friend placed Silver 1. And I, Gold 1, placed Silver 4. Though we all played every single one of our placements together. I'm 99% sure it's because of the games from last season, even though I technically never actually placed.

Hope this helps.