insufficient material, but I flagged to win?

en.lichess.org/tIv4TvBh/black

Is this a bug? Black does not have winning material, but wins the game.

There is a mate if white underpromotes into a knight: W:Ka1,Nb1 B:Kb3, Bb2

Yes, there is a mate :-)

Wow, I didn't see it. That is almost puzzle like. Thanks! :)

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