Nesting tags is invalid HTML

P.S. In case this is useful to anyone else, here’s a temporary workaround. This plugin uses the list name as the checkbox label, but I don’t have multiple lists, so I wanted a simple checkbox label: “Subscribe to our newsletter”.

// Move Constant Contact opt in before Post Comment (submit) button with clean HTML
function pd_cc_optin() {
	if ( ! function_exists( 'constant_contact' ) ) return;
	$settings = constant_contact()->get_settings();
	// Remove from 'comment_form' (fires after submit button)
	remove_action( 'comment_form', [ $settings, 'optin_form_field_comment' ] );
	// Prepend clean opt-in HTML before the submit button (works for both logged-in and logged-out)
	add_filter( 'comment_form_submit_field', function( $submit_field ) use ( $settings ) {
		// Respect the plugin's own "show on comment form" setting and API connection check
		if ( ! $settings->check_if_optin_should_show( 'comment_form' ) ) {
			return $submit_field;
		}
		if ( ! constant_contact()->get_api()->is_connected() ) {
			return $submit_field;
		}
		// Get configured lists
		$list_ids = constant_contact_get_option( '_ctct_optin_list', [] );
		if ( empty( $list_ids ) ) {
			return $submit_field;
		}
		// Get the label (falls back to default)
		$label = constant_contact_get_option( '_ctct_optin_label', '' );
		if ( empty( $label ) ) {
			$label = __( 'Subscribe to our newsletter', 'constant-contact-forms' );
		}
		// Build clean HTML with first configured list
		$list_id = reset( $list_ids );
		$html = sprintf(
			'<p class="ctct-optin-wrapper">
				<label for="ctct_optin_%1$s">
					<input type="checkbox" value="%1$s" class="checkbox" id="ctct_optin_%1$s" name="ctct_optin_list[]">
					%2$s
				</label>
			</p>',
			esc_attr( $list_id ),
			esc_html( $label )
		);
		return $html . $submit_field;
	});
}
add_action( 'init', 'pd_cc_optin', 20 );