Why does Stockfish miss mate in 5 with only two legal moves?

Question

I'm interested to understand what is going on for Stockfish on Lichess here. There are only two legal moves for Black, and it gives the position mate-in-14 for white.

When asked to play its recommended move (using Lichess "practice with computer"), it chooses Kb8 and gives the position mate-in-4 for white.

I'm very surprised that it couldn't see the mate in 4 from one half move further away, given only two moves for black to consider. And especially given that the move it chose as the best for black was on the path to the mate in 4.

Are there any insights that would help understand its miscalculation?

(Hmm, actually maybe what is going on is it's actually mate in 14, but the "practice with computer" mode is not choosing the best possible move, and e1=Q would have been stronger. In which case I wonder how to get a stronger practice mode...).

Answer 1

There is no mate in 4. Black's best move is 1...e1=Q, after which White goes 2. Bg2+ Kb8 3. a7+ Kc8 4. a8=Q+ with a decisive advantage, but no immediate checkmate. M14 is correct, White takes several moves after the promotion to mate.

You might be seeing some issue with tablebases, since if Black's goal is to prevent the zeroing move as long as possible then 1...Kb8 is the best move.