I am able to reproduce this bug and I think we should fix it.
- create a group that accepts email from any email address
- create some custom user fields
- email the group set up in (1) from an email address not already associated with an account on the site, which creates staged user with that email address
- create an account using same email address as staged user
- when prompted, complete the user fields
- go to user profile and see that the user fields are not filled
When you create an account using an email address that is not already connected to a staged user account the custom user fields are filled as you’d expect.