It would seem that the game was not ended in a normal manner. Your opponent, who apparently resigned in a winning position, (now) has the red message "This account violated the Lichess Terms of Service" on their profile. I am guessing that the game was actually terminated by the server and, for rating purposes, aborted.
I may be wrong. Others should correct me if that is inaccurate.
EDIT: Cross-posted with sheckley666.