Inconsistent error: Request method 'POST' not supported

Question

Using Spring REST API [Spring MVC]

Scenario:

when request comes to the EmployeeController , it is forced to forward the request/response to another URI,if it falls under a specific logic. The controller method has RequestMapping set with 'RequestMethod.POST' and the destination controller- SpecialController has the method named invalidRequest() that has RequestMapping set with 'RequestMethod.GET'

EmployeeController:

@RestController
@RequestMapping(value = "/employee")
public class EmployeeController {

    String res = null;

    @RequestMapping(value = "/update", method = RequestMethod.POST)
    public String updateEmployeeDetails(HttpServletRequest httpRequest, HttpServletResponse httpResponse,
            @Valid @RequestBody Employee emp) throws ServletException, IOException {
         // based on logic, forward the request to a different controller that handles invalid request

    if( ...) { // condition checking
        RequestDispatcher requestDispatcher = httpRequest.getServletContext().getRequestDispatcher("/invalidRequest");
        requestDispatcher.forward(httpRequest, httpResponse);
    
    }

   if(..someother condition..) {
        String res = "something";
   }
return res;

Destination Controller:

@RestController
    @RequestMapping(value = "/invalidRequest")
    public class SpecialController {

        @RequestMapping(value = "", method = RequestMethod.GET)
        public String invalidRequest(HttpServletRequest httpRequest, HttpServletResponse httpResponse) {
            httpResponse.setStatus(401);
            return "invalidRequest";
        }

    }

Question: Inconsistency Problem [Actual Problem]:

In 90% of the times, this is working, but a few times very rarely, I get the below error. If I am getting this error always, then it would have made some meaning and I would have the below mentioned 'possible fix' But since it is working most of the times, and not working only sometimes, I need your help in finding out why?

> org.springframework.web.HttpRequestMethodNotSupportedException:
> Request method 'POST' not supported
>         at org.springframework.web.servlet.mvc.method.RequestMappingInfoHandlerMapping.handleNoMatch(RequestMappingInfoHandlerMapping.java:198)
>         at org.springframework.web.servlet.handler.AbstractHandlerMethodMapping.lookupHandlerMethod(AbstractHandlerMethodMapping.java:286)
>         at org.springframework.web.servlet.handler.AbstractHandlerMethodMapping.getHandlerInternal(AbstractHandlerMethodMapping.java:233)
>         at org.springframework.web.servlet.handler.AbstractHandlerMethodMapping.getHandlerInternal(AbstractHandlerMethodMapping.java:56)
>         at org.springframework.web.servlet.handler.AbstractHandlerMapping.getHandler(AbstractHandlerMapping.java:300)
>         at org.springframework.web.servlet.DispatcherServlet.getHandler(DispatcherServlet.java:1101)
>         at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:916)
>         at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:876)
>         at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:961)
>         at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:863)

possible fix if the error was consistent:

@RestController
    @RequestMapping(value = "/invalidRequest")
    public class SpecialController {

        @RequestMapping(value = "", method = RequestMethod.GET)
        public String invalidRequest(HttpServletRequest httpRequest, HttpServletResponse httpResponse) {
            httpResponse.setStatus(401);
            return "invalidRequest";
        }
        
        @RequestMapping(value = "", method = RequestMethod.POST)
        public String invalidRequest2(HttpServletRequest httpRequest, HttpServletResponse httpResponse) {
            return invalidRequest(httpRequest, httpResponse);
        }

    }