Nevermind, solution found. I reordered the way css was loaded. Rather than load my editor CSS first, I loaded it last. I undersestimated how comprehensive the default editors CSS was, so I wasn’t seeing any of my “overiding” CSS. I changed the if statement to this:
$content_css = get_stylesheet_directory_uri() . '/bd-editor-style.css';
if ( isset( $mce_init[ 'content_css' ] ) ) {
$content_css_new = $mce_init[ 'content_css' ].','.$content_css;
}
$mce_init[ 'content_css' ] = $content_css_new;