Series complete after 1 match ?

I won a match to get to 100 lp and start my promos to the next division. I lost my first match (out of supposedly 3) and went down to 78 lp which is normal but I also "completed my series." Not sure if this is intended due to low mmr or if it's a bug. I've checked if it's just a visual glitch but no, I'm not up for promotion anymore in my league