Unfortunately this type of situation is tough to deal with because reducing the LP lost for a team that had a leaver can be abused. If a team is losing they may begin to harass a teammate until that teammate quits, allowing the team to get a reduced LP loss for the game. This promotes a more negative gameplay environment.
J0o0's post on when the LP losses apply is also spot on so I won't resay what he already pointed out.
I feel like all of us have had this issue at some point or another in ranked as a whole. The best thing to do is keep your head up and move on. Here is another thing to help: Negative Players, whatever the reason, are more likely to be on the enemy team then yours! I am defining negative by players who are AFK/Toxic/INT'ing, NOT having a bad game.
Lets take an example game where there is a negative player. There are 9 other people in the game that you have no control over. This is assuming you are not negative yourself and conduct yourself with thought to the Summoners Code. Of these 9 players, 5 of them are on the enemy team and 4 of them are on yours (as you take up one of the 5 spots on your team). This means that with 9 people in the game, if there is a negative player there is roughly a 55% chance that said negative player is on the enemy team and roughly a 44% chance that they are on your team! While that still means there is a negative player, that 11% difference can be a big deal.
I'd personally love it if my win rate went up 11%... It would be at like... 12% but that's a different story!
Best advice is just keep your head up, move onto the next game, and remember that this is NOT the norm when there is a negative player!
Best of luck on the Rift,
Kuthillick