I've had a bug similar to this before where I was actually in my BO3 and had already played my first match. Both the client and lolking seemed to glitch, because when I played and lost the 2nd game a few days later, it showed a drop in LP, both in the client and on lolking. As you know, you shouldn't lose LP in the middle of a promo series. I then played the 3rd game, won, and got promoted. The dip in LP still shows on my lolking chart to this day even though the 1 loss was game 2 of a promo series.
I had another instance where I had already played my first 2 promo games in a series and then didn't play for about a week. When I got on to play the last game, the client and lolking were both showing that I was not in a promo series, and had 80-some LP. I was pretty bummed because I thought I was playing game 3 for promotion. I thought I was going crazy. I won the game, and got promoted. It was game 3 after all.
TLDR: Lots of promo series glitches. I don't know what happened in your particular instance, but it's probably not too uncommon for strange shit like that to happen.