filter just a portion of plugin function

Looks like that isn’t the filter the want. It runs right at the end of the BBPress bbp_get_breadcrumb function which is after the stuff you want to change.

You can change just the separator with bbp_breadcrumb_separator and it looks like before and after can be set with bbp_get_breadcrumb_pre. In fact, it looks like you should be able to use that last filter for all of them.

I don’t run BBPress so this is untested but what you want probably looks like:

function custom_bbp_breadcrumb() {
    $args['before']  = '<div class="bbp-breadcrumb">';
    $args['after']   = '</div>';
    $args['sep']     = __( '&rsaquo;', 'bbpress' );
    return $args;
}
add_filter('bbp_before_get_breadcrumb_parse_args', 'custom_bbp_breadcrumb' );

What you were doing with your filter function was completely erasing the input. You created a brand new array inside your filter function and sent that new array back out. What the modified filter does is take the input, alter it, and give it back with only some parts changed and everything else left alone.