Is it a bug that i got -20 for winning a game that i got disconnected in????

I just had a game where my shit lan went out (i pay 80€/month). Luckily my team was doing good and i wasnt feeding so i managed to reconnect 3 minutes later and win the game. That game counted as a "leaver" and it gave me a -20lp rating. For what??? Is that a bug? If not, its completely %%%%%%ed. Do all people lose their lp when they disconnect? Why even reconnect then?
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