It is a consequence of the Betza parser not printing any error messages. In [W?fB] the f and B are conflicting input: f says you should keep moving in the same direction as the W step, while B says you must move diagonally. So it is an invalid specification, with undefined result.
What happens in practice is that the [...] notation is implemented by a pre-processing step that replaces it with the original XBetza notation using the a modifier. It thus uses the first step as the atom, tests whether a range-toggle is required for deciding it should be prefixed with a or ya, and takes the modifiers on the second leg. So [W-fB] becomes yafW, which is what [W-fR] would become too. The B or R are simply ignored (apart from detecting that they are sliding moves, so that the y will be prefixed). The preprocessor is not smart enough to understand directionality of the second-leg atom, nor that non-matching directionality would require addition or removal of an s to get the correct bend. It treats all second-leg atoms like they are K or Q, and you have to supply the s yourself.
It is a consequence of the Betza parser not printing any error messages. In [W?fB] the f and B are conflicting input: f says you should keep moving in the same direction as the W step, while B says you must move diagonally. So it is an invalid specification, with undefined result.
What happens in practice is that the [...] notation is implemented by a pre-processing step that replaces it with the original XBetza notation using the a modifier. It thus uses the first step as the atom, tests whether a range-toggle is required for deciding it should be prefixed with a or ya, and takes the modifiers on the second leg. So [W-fB] becomes yafW, which is what [W-fR] would become too. The B or R are simply ignored (apart from detecting that they are sliding moves, so that the y will be prefixed). The preprocessor is not smart enough to understand directionality of the second-leg atom, nor that non-matching directionality would require addition or removal of an s to get the correct bend. It treats all second-leg atoms like they are K or Q, and you have to supply the s yourself.
Perhaps this can be improved one day.