It is possible that your browser and its extensions add some content within the source code of the email. To check this, go to Exit > Send email & click on Source.
As in the following example, some content can be added to the HTML code.
As a consequence, an email subject line is added as soon as there is some custom HTML. Emails are triggered when they have content + subject line.
To avoid this, make sure that you delete the title & all the content in Source, then save it.
Without extensions, this is how the generic HTML should look like.