How to handle exceptions in spring web flux functional end point?

Question

I want a global exception handler for the inputs in spring boot web flux functional end point.

I have written exception class by extending runtime exception and handler with @component scan. My problem is even though an exception occurred handler will not handle it.

Exception class

@ResponseStatus(HttpStatus.NOT_ACCEPTABLE)
@Data
public class InputValidationException extends RuntimeException{

    private final String input;
    private static final int errorCode = 400;

    public InputValidationException(String message, String input) {
        super(message);
        this.input = input;
    }
}

Exception Handler Class

@ControllerAdvice
public class InputValidationExceptionHandler {

    @ExceptionHandler(InputValidationException.class)
    public ResponseEntity<InputValidationResponse> handleException(InputValidationException ex) {
        InputValidationResponse response = new InputValidationResponse();
        response.setErrorCode(ex.getErrorCode());
        response.setMessage(ex.getMessage());
        response.setInput(ex.getInput());
        return  ResponseEntity.badRequest().body(response);
    }
}

My functional end point

@RouterOperation
    @Bean
    public RouterFunction<ServerResponse> userForgetPassword(UserHandler handler){
        return RouterFunctions.route().
                POST("/api/users/forgetPassword/{email}",
                        accept(APPLICATION_JSON), handler::sendOtp)
                .onError(InputValidationException.class,sendOtpExceptionHandler())
                .build();
    }

    public BiFunction<Throwable, ServerRequest, Mono<ServerResponse>> sendOtpExceptionHandler(){
        return (err, req)->{
            InputValidationException ex = (InputValidationException) err;
            InputValidationResponse response = new InputValidationResponse();
            response.setInput(ex.getInput());
            response.setMessage(ex.getMessage());
            response.setErrorCode(ex.getErrorCode());
            return ServerResponse.badRequest().bodyValue(response);
        };
    }

Service method

public Mono<Object> sendOtp(String email){
    Mono<String> otpMono = generateOtp();
    return patternMatches(email,EMAIL_REGEX)
                        .handle((valid, sink) -> {
                            if(valid)
                                sink.next(email);
                            else
                                 sink.error(new InputValidationException("Email not valid", email));
                        })

Response of the API

{
    "timestamp": 1652806523422,
    "path": "/api/users/forgetPassword/lakmaldasun2011",
    "status": 400,
    "error": "Bad Request",
    "requestId": "845f18f6-1"
}

Can anyone tell me what is the problem here to not to handle the exception?

Thanks.