Getting "Leaver" even though I helped carry the game during placements?

I decided to play Senna this game as I have been playing enchanter supports for a while and I wanted to switch over to a carry support. Me and the Lucian are duo queued. Me and him did great in lane and we snowballed our lead until I DCed because of ping problems. Our Garen lost lane to Mordekaiser and it wasn't going too well for our team. I was gone for around 4 minutes, and I came back to the game ready to play. I provided vision to our team, helped dish out damage during team fights, and was constantly pinging objectives in case my team was unaware they could be doing Baron or Elder. I am very disappointed that the system has decided to put me as a "Leaver" heck I could've just quit the game and my team would've lost if I wasn't playing. This needs to be fixed.