WooCommerce uses the Emogrifier class to add inline styles to the email. It is a small utility to convert css to in-line styles.
However, while parsing your template, this breaks some of styles you have made.
That is the reason why a few of the styles got broke.
Solution is to change the styles for that block (You can re-design that block alone)
If it keeps breaking, then here is a quick workaround
Around line 462, you will find
$content = $emogrifier->emogrify();
Change this to
//$content = $emogrifier->emogrify();
This would mean that the content does not pass through emogrifier utility.