I won my last promo game and didn't promote???

So. I just finished playing my last promo/series game to Silver 2 in Ranked Flex. I won the first, lost the second and just won the third.

I, however, had a lot of connection issues after about 15 minutes into the game and my ping was going sky-high. I left the game and then reconnected to see if that would solve anything. It did and the game went on smoothly and we made quick work of them. Not because of me anyway, as I had lost the gold and xp advantage when I had to afk and see if I could fix the connection problem. Therefore I ended the game several lvls below my teammates and with a D- (even though I had positive stats).

After that I had honored a teammate, I saw the normal little pop-up window that tells you that you have either failed or promoted, yet it said I had failed when I won the game? I was roughly afk for 5 minutes before I could reconnect.

Was that reason enough to take the win away from me and ending my series? Or was it just a bug?

Not that big of a deal as I do not care as much about Ranked Flex as I do about Solo/Duo but it was still very weird and would have made me really angry if it was a higher elo series that I lost because of it.