Opponent leaves the game and then I leave too

I was playing a casual blitz game, at a certain point (quite early in the game) my opponent disconnects, I wait 10 seconds and he doesn't reconnect, so I leave the game.
I come back after more or less 20 seconds and I find out that he moved and I lost the game because he claimed his victory. Usually I am the one that claims victories because my opponents leave, so this time it was unexpected. It would be nice if I get a notification for my opponent's move.

Has this happened before to people? Thanks in advance for your feedback! Have a nice day!

What exactly do you mean "Left the game"? Did you close the tab? If you close the tab, lichess has no way of knowing that you did not rage quit, so it will forfeit the game to your opponent. Yes, this happened to me once in a classical game, and ever since then, I just open another lichess tab and wait for the move on the other tab. It will say on that tab"waiting for opponent" , and when they move, it will say "your turn". I leave the tab up always though, otherwise the other player can claim victory. Hope this helps! If you left the tab open, were you playing another game? This can also result in forfeiting the game.

Thanks for your reply! I was playing with my phone (using the browser, not the app) and if I remember correctly I pressed the home button without closing the Chrome app. It's not a big deal, it was just a casual game with a lower rated player. Thank you for the explanation, anyway!