There are always workarounds to implement the sorting:
Add this function:
function se_sort_tags_alphabetically( $tags = array() ) {
if ( $tags ) {
$sortData = $sorted = array();
foreach ( $tags as $i => $tag ) {
if ( empty( $tag->name ) ) continue;
$sortData[$tag->name] = $i;
}
krsort($sortData);
foreach ( $sortData as $index ) {
$sorted[$index] = $tags[$index];
}
$tags = $sorted;
}
return $tags;
}
Then use:
$post_tags = se_sort_tags_alphabetically(get_the_tags());
Hope that helps.