New CiviMail does not use the 'CMS editor' set in 'Display settings'
My understanding is that the new CiviMail approach necessitated disabling the "use CMS default" option for WYSIWYG.  The new preferred approach is CiviCRM extensions to implement other WYSIWYG editors - it took about an hour to create the first WYSIWYG extension for CiviCRM, and others can build even quicker, using that extension as a guide.  You can find it here: https://github.com/relldoesphp/com.aghstrategies.tinymce