Spring Boot Websockets and Angular/Stompjs: No DataFrom Subscription

Question

I have a Spring Boot/Angular 17 application using StompJS. I am NOT using rx-stomp.

This is my controller on the backend:

@RestController
@MessageMapping("idle")
public class IdleWebsocketController {

    private AppUserService userService;
    
    public IdleWebsocketController(AppUserService userService) {
        super();
        this.userService = userService;
    }

    @MessageMapping("resetTimer")
    @SendToUser("/topic/resetTimer")
    Long sendResetIdle(Principal principal) {

        String userId = principal.getName();
        userService.updateActivity(userId);
        
        Long time = new Date().getTime();
        return time;
    }
}

On the UI side, I can publish to /idle/resetTimer. I try to subscribe to the resetTimer topic with this code:

    const callback : messageCallbackType = (message) => {
      console.log(`Message: ${JSON.stringify(message)}`);
    };

    return of(undefined)
      .pipe(
        tap(() => {
          this.websocketService.subscribe("/user/topic/resetTimer", callback);
        }),
      );

(The of(undefined) is because I need to run the several functions together in a combineLatest call elsewhere, so I need to return an Observable<void>).

I don't see any errors suscribing, but I am getting no results back to the UI side once the timer is updated on the API side.

Am I subscribed incorrectly?