Calendar Flickering in Updates

useEffect(() => {
  if (pickedService.length > 0 && gender !== "") {
    pickedService.forEach((service) => {
      let event = calendarRef.current!.control.events.find(
        service.extra.appointment_id ?? service.service.id_service
      );

      if (service.extra.toDelete === false) {
        if (event !== null) {
          let clientName = setIsEditingEventPosition(true);
          calendarRef.current!.control.events.update({
            id: service.extra.appointment_id ?? service.service.id_service,
            text: service.service.id_service,
            start: service.date!["start"],
            end: service.date!["end"],
            resource: service.resource.id,
            cssClass: service.extra.appointment_id
              ? "draft_selected_event"
              : "draft_event",
            barHidden: true,
            tag: {
              marker: service.service.ec_color_light,
              client:
                (!name ? "Passaggio" : name) +
                " " +
                (surname ??
                  (surname ||
                    (watchFields.id_cus === undefined
                      ? gender
                      : extractGenderFromID(watchFields.id_cus!)))),
              service: service.service.name,
              writtenNote: note,
              note:
                service.extra.note !== undefined && service.extra.note !== "",
              heart: service.extra.worker_lock !== "0",
              clock: service.extra.time_lock !== "0",
            },
          });
        } else {
          setIsEditingEventPosition(true);
          const event = new DayPilot.Event({
            id: service.extra.appointment_id ?? service.service.id_service,
            text: service.service.id_service,
            start: service.date!["start"],
            end: service.date!["end"],
            resource: service.resource.id,
            barHidden: true,
            cssClass: service.extra.appointment_id
              ? "draft_selected_event"
              : "draft_event",
            tag: {
              marker: service.service.ec_color_light,
              service: service.service.name,
              client:
                (!name ? "Passaggio" : name) +
                " " +
                (surname ??
                  (surname ||
                    (watchFields.id_cus === undefined
                      ? gender
                      : extractGenderFromID(watchFields.id_cus!)))),
              writtenNote: service.extra.note,
              note:
                service.extra.note !== undefined && service.extra.note !== "",
              heart: service.extra.worker_lock !== "0",
              clock: service.extra.time_lock !== "0",
            },
          });
          calendarRef.current?.control.events.add(event);
        }
      }
    });
  } else {
  }
}, [pickedService, gender, deferredQueryName, deferredQuerySurname]);

When i run this code, everytime the useffect is performed the whole calendar events flickers, not just those whom i'm dealing with. Why?