You have more informations by putting your mouse on your elo.
On that game you:
- win against the player which finished 3rd which gave you +3.52 elo
- tied against the player which finished 1st which gave you -3.62 elo
As a result, you get -0.1 elo.
You are expected to win against players which have a lower elo than you.
In some games that's unfair, but in games where ties are really rare like it seem the one you just shared, it's just a choice they made imo.
Them being able to get a tie is a reward which isn't a bad reasonning imo.
About leaving games:
Normally, if you get as a result negative elo overall it's set to zero if someone left.
And your gain of elo is multiplied by the progression of the game (multiplied by a number between 0 and 1).
One point of logic is that the elo of low ranked players doesn't matter much. Maybe even them playing one more game this season is a sign they should be slightly higher. And if a player have too much elo, it's not a big deal.
On the contrary high ranked players must be at the right place. If you gave X free elo by having someone leaving turn 1, it's not a sign they are stronger than others, they were just lucky someone left early.
Though, I'm not even sure that would be bad to just not count others players in case someone left, and give you a 1v1 win against the player who left.
