The bug’s only 4 years old so you wouldn’t want to rush them would you?! A workaround is to leave out the wp_style_add_data()
and use the 'style_loader_tag'
filter:
add_filter( 'style_loader_tag', function ( $tag, $handle ) {
if ( $handle == 'purecss-all-not-ie' ) {
$tag = "<!--[if gt IE 8]><!-->\n" . $tag . "<!--<![endif]-->\n";
}
return $tag;
}, 10, 2 );