Mate Bug?

[NewbieChess]

I'm playing and testing a couple engines, and then I came across some weird moves.

The engine would see mate in one, but wouldn't play it. If the opponent's move doesn't prevent mate, it plays another random move. The cycle repeats and repeats.

I did a couple tests against those engines (Sloppy, Slow Chess Blitz, MicroMax) and they all seem to display that behavior.

Is this a bug or a variant of chess I'm not aware of?