CKeditor stripping allowed style attribute
Your problem comes from the "Limit allowed HTML tags" option. According to this post (for Drupal 7, but the same logic applies on D8), setting this option triggers XSS filter in order to remove dangerous tags.

As for Drupal 8, it uses the core Filter module and calls filterXss() method in \Drupal\editor\EditorXssFilter\Standard class. First line of this method is:

$dangerous_tags = ['script', 'style', 'link', 'embed', 'object'];


Hence, your style tag will always be removed with the "Limit allowed HTML tags" option set.

If you really wish to use inline style, you have to either:


unset the "Limit allowed HTML tags" option (which I do not recommend depending on the kind of users you give access to "Basic HTML" text format)
or use "Full HTML" text format


Best solution would be to rely on classes and styling in CSS stylesheets and inline styling should be used sparingly.

Hope this helps.