This has been happening to me recently and I am curious if anyone can shine some insight here...
1. I had won my s2 promos starting with (S2, 0LP)
2. Won my next match with an S, (S2, 21LP)
3. Lost the next match (-18 was recorded in postgame) however the client calculated me back to (S2, 0LP)
Either the league client's calculator is broken or I should still have 3LP.
This is important because if you lose 2 matches at 0LP you get demoted, rather if you have the 3LP buffer you tend to still have an extra game to save yourself.
Has this happened to anyone before? I logged a bug complaint, hoping to receive an answer.