Hey Kevin and all,
We coded a fix. Paste the following into your functions.php
add_action( 'init', '____action_function_name' );
function ____action_function_name() {
// Include Extension files
require_once dirname( INTEGRATE_ELEMENTOR_MAILSTER_PATH ) . '/includes/class-mailster-action-after-submit.php';
// Instantiate the action class
$mailster_action = new Mailster_Action_After_Submit();
// Register the action with form widget
\ElementorPro\Plugin::instance()->modules_manager->get_modules( 'forms' )->add_form_action( $mailster_action->get_name(), $mailster_action );
}