Looking at the contents of class-wp-editor.php, it looks like tiny_mce_before only triggers if you are not using the Teeny version of the editor:
/*
* For people who really REALLY know what they're doing with TinyMCE
* You can modify $mceInit to add, remove, change elements of the config
* before tinyMCE.init. Setting "valid_elements", "invalid_elements"
* and "extended_valid_elements" can be done through this filter. Best
* is to use the default cleanup by not specifying valid_elements,
* as TinyMCE checks against the full set of HTML 5.0 elements and attributes.
*/
if ( $set['teeny'] ) {
/**
* Filters the teenyMCE config before init.
*
* @since 2.7.0
*
* @param array $mceInit An array with teenyMCE config.
* @param string $editor_id Unique editor identifier, e.g. 'content'.
*/
$mceInit = apply_filters( 'teeny_mce_before_init', $mceInit, $editor_id );
} else {
/**
* Filters the TinyMCE config before init.
*
* @since 2.5.0
*
* @param array $mceInit An array with TinyMCE config.
* @param string $editor_id Unique editor identifier, e.g. 'content'.
*/
$mceInit = apply_filters( 'tiny_mce_before_init', $mceInit, $editor_id );
}
If the config can work for both Tiny and Teeny, you could hook on both filters to modify the configuration.