Timed out as white left the game (in a classical game) even with tab open

This was the game -

I was completely winning and my opponent was taking some time to move so I just switched tabs to check my email. This was less than a minute. When I came back to the original tab, it showed black to have claimed the victory as white left the game. To be clear, the Lichess game tab was open the whole time.

I also use Firefox, which sends me a notification if I try to close a Lichess page with a game on asking me to confirm "Do I want to leave page, information may not be saved". So if I am trying to close a tab with a live game, I need to doubly confirm.

While I am not sure if my internet disconnected, if that was the case, it could only be have for a maximum of 30-40 seconds - as I was reading an email for that long. However, I do not think it disconnected. Even if it did disconnect - There was only 43 seconds that had passed (the timing graph shows this). It seems too little time - in a classical game (as I remember being made to wait for up to 25 seconds in a blitz game).

Could someone with the information as the how this is possible clarify?

With a bullet and blitz game a short cut off time may be justifiable - but in a classical game it hurts, as you have invested a lot of time (especially when you are winning).

Thank you.

Just an update:

In another game, my opponent left the game after move 5 and I had to wait 110 seconds to claim a victory or a draw (I waited for longer and my opponent came back online - would have called a draw if my opponent did not come back)

Edit: it was a classical game with the same time controls as the game above (30+30)