Hi Nim,
Welcome to BuddyDev.

Thank you for asking it. It is a problem with BuddyPress.

Here is a simple fix for register page

/**
 * Force to use assigned Page template with BuddyPress Register page
 *
 * @param $template
 *
 * @return string found template
 */
function buddydev_register_template_fix( $template ) {

	if ( bp_is_register_page() ) {
		$template_slug = get_page_template_slug( buddypress()->pages->register->id );

		if ( $template_slug && ( $located = locate_template( array( $template_slug ), false ) ) ) {
			$template = $located;
		}
	}

	return $template;
}
add_filter( 'template_include', 'buddydev_register_template_fix', 100 );

If you put it in your bp-custom.php or theme’s functions.php(may be child themes’). It will force WordPress to use assigned template for the register page.

Hope that helps.

Regards
Brajesh