I’m guessing a bit here. Do you have DiscourseConnect enabled between WordPress and Discourse? If so, users are probably clicking the “Log in” button on your Discourse site to initiate the sign up and not clicking a “Sign Up” button.
Assuming the above is correct, if you have the “Create or Sync Discourse Users on Login” option enabled on the WP Discourse DiscourseConnect Provider options tab enabled, that could likely cause a different login flow for users who signup through a Wishlist Member registration link than for users who signup by clicking the “Log in” button on Discourse. I could be way off base here though if that’s not how you have things configured.