No, black did not leave the game.

What counts as leaving the game, and how long does a player have to return?

In Thailand the concept of reliable infrastructure has not yet been invented, so I do regularly lose connection for brief periods of time. But so far as I know it only counts as leaving a game when a player leaves for a relatively lengthy period of time. That is not what happened here as I did not even have enough to leave for a lengthy period. I was surprised and a bit frustrated to see the game forfeited after reconnecting a few seconds later.

I've had this too - a temporary connection issue and after barely 30 seconds the game is gone.

I always wait at least a few minutes (longer, depending on the length of the game) before clicking that button, but there should be a grace period for the less fortunate souls of lichess.

this is strange. when opponent disconnects, that button waits 2-3 minutes before showing up , but when i disconnect for 10 seconds, opponent has already claimed his win. why is this ?