Yep, a bug.
The fix is to add all the fields to the list (probably some where left during the transition from "fields" to "custom fields").
In other words, in Register.controller.php, find:
		$possible_strings = array(
			'birthdate',
			'time_format',
			'buddy_list',
			'pm_ignore_list',
			'smiley_set',
			'personal_text', 'avatar',
			'lngfile',
		);
		$possible_ints = array(
			'pm_email_notify',
			'notify_types',
			'id_theme',
		);
and replace with:
		$possible_strings = array(
			'birthdate',
			'time_format',
			'buddy_list',
			'pm_ignore_list',
			'smiley_set',
			'personal_text', 'avatar',
			'lngfile', 'location',
			'secret_question', 'secret_answer',
			'website_url', 'website_title',
		);
		$possible_ints = array(
			'pm_email_notify',
			'notify_types',
			'id_theme',
			'gender',
		);
			
				I forgot to xref the bug report: https://github.com/elkarte/Elkarte/issues/1857
			
			
			
				Fix pushed: https://github.com/emanuele45/Dialogo/commit/d66e0fa831ee5a38ac8f9d701da6fb1ed31f4205
Moving the topic while I have it on the radar.