You are right; the move diagrams have no provision for 'backup moves'. The highlights in move diagrams are still shown by the old move generator, not the one used in the AI. (The requirements are different: in move diagrams you want to highlight potential capture-only moves, even when the piece is shown on an empty board.) This old move generator only shows the primary moves.
I am not sure how this would have to be fixed in general. Perhaps the backup moves should always be shown, but with different marker symbols for indicating they are backup moves.
You are right; the move diagrams have no provision for 'backup moves'. The highlights in move diagrams are still shown by the old move generator, not the one used in the AI. (The requirements are different: in move diagrams you want to highlight potential capture-only moves, even when the piece is shown on an empty board.) This old move generator only shows the primary moves.
I am not sure how this would have to be fixed in general. Perhaps the backup moves should always be shown, but with different marker symbols for indicating they are backup moves.