Bug?

No, is En Passant (en.wikipedia.org/wiki/En_passant) En passant is a move in chess. It is a special pawn capture that can only occur immediately after a pawn makes a move of two squares from its starting square, and it could have been captured by an enemy pawn had it advanced only one square. The opponent captures the just-moved pawn "as it passes" through the first square. The result is the same as if the pawn had advanced only one square and the enemy pawn had captured it normally.

The en passant capture must be made on the very next turn or the right to do so is lost. En passant capture is a common theme in chess compositions.

The en passant capture rule was added in the 15th century when the rule that gave pawns an initial double-step move was introduced. It prevents a pawn from using the two-square advance to pass an adjacent enemy pawn without the risk of being captured.

I hope you liked my explanation.

This isn't a bug but it is still worth mentioning it. The first time a computer played this I also thought it was a bug but it isn't. This move is called En Passant and is one of the pawn's special move. If you are playing with the white pieces then if your pawn is in the 5th rank and afterwards your opponent moves his pawn two squares and lies next to your pawn then En Passant is applied, which means you can capture the pawn even though it is not one square diagonally to your pawn.

Hope this may help you understand better.

This is in the lichess F.A.Q. (Which technically means you shouldn't post this). While you might not be aware of it, somebody calling "en passant" a bug or the player playing it OTB a cheater is a tale as old as time. The purpose of en passant is to stop cheap endgames and I guess middlegames where one pawn can pass another because of this, hence the name, which translates to "in passing".