Hi Kojo,
I have checked and it is default behavior that is adds a new user when option enabled to allow user to add a listing.
When preview/submit listing action triggered it generates listing entry in database and for that listing also creates new user based on username & email entered.
When new user is created it also creates Affiliates user via AffiliateWP plugin.
Thats why you receiving email for new user, new affiliate registration etc.
I only found issue here is empty username & email in email. I have added validation to skip new user for invalid username/email.
FYI: i have installed Email Log plugin to debug all the emails, you can delete this plugin after you done with test.
Please check and let us know.
Thanks,
Kiran