Would you try with this bean configuration:
@Bean
public ServletContextInitializer initializer() {
return new ServletContextInitializer() {
@Override
public void onStartup(ServletContext servletContext) throws ServletException {
servletContext.addListener(new CustomJacksonDateFormatListener());
servletContext.setInitParameter("org.camunda.bpm.engine.rest.jackson.dateFormat",
"yyyy-MM-dd'T'HH:mm:ss");
}
};
}