Does not work for multisite registration

The plugin does not work (no captcha form shown) for multisite signup pages, e.g:

example.com/wp-signup.php

This page is always redirected to when trying to signup with regular page (which is well supported by the plugin):

example.com/wp-login.php?action=register

The code that causes this issue located in includes/class-core.php:

                if ( Math_Captcha()->options['general']['enable_for']['registration_form'] && ( ! is_user_logged_in() || (is_user_logged_in() && ! Math_Captcha()->options['general']['hide_for_logged_users'])) && $action === 'register' ) 
        {
            // Check IP rules
            if (Math_Captcha()->options['general']['ip_rules'])
            {
                $geo = new MathCaptcha_GEO();
                if ($geo->checkIP_in_List(false, Math_Captcha()->options['general']['ip_rules_list'])) return; // Dont show captcha
            }
            // Check GEO rules
            if (Math_Captcha()->options['general']['geo_captcha_rules'])
            {
                $geo = new MathCaptcha_GEO();
                if (isset(Math_Captcha()->options['general']['hide_for_countries'][ $geo->getCountryByIP(false) ])) return; // Dont show captcha
            }
            
                        add_action( 'register_form', array( $this, 'add_captcha_form' ) );
                        add_action( 'register_post', array( $this, 'add_user_with_captcha' ), 10, 3 );
                        add_action( 'signup_extra_fields', array( $this, 'add_captcha_form' ) );
                        add_filter( 'wpmu_validate_user_signup', array( $this, 'validate_user_with_captcha' ) );
                }

It appears that wp-signup.php does not use $action === ‘register’