6.a4 prompts notice that Black may capture en passant on a3?
Well, the person who recently responded has obviously been suspended. In any case, this is an NCM bug. The board on the engine output always shows the en passant square regardless of whether or not it’s vulnerable to capture.
There’s some cleaning up of en passant we need to do on NCM. From my current understanding, the en passant square should always appear in the FEN even if there is no attacker, but we should not display the X on the board unless there is an attacker.