Low LP gain with high WR/MMR
So i've been grinding rank recently and at first it was going amazing, had a 14 game winstreak, but then it kinda evened out and am getting a balanced amout of wins/losses, (around 64% winrate) with no big losing streaks or no problems being "stuck" in a rank. My question is however, with a 64% winrate and mmr that is plat III-plat II, why would I still gain only 18lp. It's not just the game that i linked, I lose 18 LP in a loss-game and haven't gained above 18 when that big winstreak ended. It's fairly obvious that I'm doing well enough in my games with participations/vision score/teamfights/objectives, etc. that i should be getting at least 22-24lp per game, just wondering if anybody can answer what the issue might be or is it another fuck-up from Riots ranking system?