Why is this move into a simple forced mate annotated dubious and not a blunder? In a recent game against a weak computer opponent on lichess, I (white) was in the following position:
[FEN &quot;Q4k1r/p1Nbbppp/1pn1p3/8/5B2/5N2/P1K1nPPP/R6R b - - 0 1&quot;]

The AI played the move Nb8 giving white a forced mate in 3, annotated by the lichess analysis board as a Nb8?! -- it looks like Bd8 is a better move by blocking the check with a supported bishop.
However, at least to my novice understanding, a move into a forced checkmate (let alone a relatively easy-to-see one in three moves) instead of a far better alternative should be a blunder, not a dubious move (which I understand as &quot;questionable but possibly having merits&quot;). Is there something I'm missing about the nature of this checkmate which makes Nb8 dubious but &quot;possibly having merits&quot;, or is it a case of wrong automated annotation?
A blunder is a game-changing move. Nb8 does not change the result of the game which is utterly and completely lost regardless of black's move. Earlier in the game black did blunder when it gave away its queen.
The reason for the &quot;!&quot; part of the &quot;?!&quot; (dubious move) notation in this case is because moving the knight opens up the line of sight of the d7 bishop.