How does the #submit property works with #ajax?
I don't think its good idea to handle form submission using the submit_form function in case of AJAX form. All the form submission logic should be handled in AJAX callback only. 
https://drupalabc.xyz/question/how-handle-form-submission-using-ajax-drupal-8

The following link has a running example of AJAX form submission in Drupal 8. I hope this helps.
Thanks.