https://support.riotgames.com/hc/en-us/articles/204010760-Ranked-Play-FAQ#h2q5
WHAT HAPPENS TO MY LP IF I LOSE MY PROMOTION SERIES?
The amount of LP that is deducted from 100 is equal to the net loss of LP from your promotional games. For example, if you win one out of 3 games, you will receive 1 LP gain and 2 LP losses. The net difference is what will be subtracted from 100.
For you, what was happening before is you won one match and lost two, so you gained LP for the win and lost LP for the 2 losses (losing ~25 LP each loss and gaining ~25 LP for the win).
In this series, you lost twice and didn't win one so you lost a whole 52 at ~26 LP each.