I was denied promotion to gold 2 after winning promos

I was in promos for g2. In a best 2 out of 3, i lost the first game, then won the second game but after the second game it said "series incomplete" and put me back to g3 70lp. This has actually happened to me multiple times now and I dont get why. Isnt it a best 2 out of 3? why is losing the first game an instant drop out of promos?

Another thing is during my second game my client crashed for 3 min. I came back and still helped my team achieve victory. Did afking the game, albeit unintentionally, make me get dropped from promos? or is this a bug?