After a user registered an account, there is no row for that user in the phpbb_profile_fields_data table. The profile field manager's grab_profile_fields_data() method returns an empty array.
However, after the user went to their UCP and updated their profile for the first time, even when not filling any field data, there is a row in the phpbb_profile_fields_data table with all fields being empty. grab_profile_fields_data() now returns an array containing the profile fields' data with the user's values all empty. This is the expected behavior.
By default, all profile fields have the field_show_novalue set to false which is why grab_profile_fields_data() returns an empty array, rather than an array populated with all the profile fields data and the user's values all empty: https://github.com/phpbb/phpbb/blob/release-3.2.8-RC1/phpBB/phpbb/profilefields/manager.php#L367
This means that, for a user who has never updated their profile, there is no way to fetch profile fields data (meaning information about the fields, not the user's values (which are obviously empty)).