The WYSIWYG editor works just like the WP main editor. There is nothing in ACF that will let you do this and I don’t think there is anything in WP that will let you do this. If you find something that will work in the main WP editor, it will also work in the ACF field.
https://wordpress.stackexchange.com/questions/85054/how-to-add-a-class-to-ul-tags-created-by-the-wordpress-editor-tinymce/85071, the last comment is pretty much what I was thinking, having a class on the wrapper element of the ACF output rather than trying to force classes into the editor.
