Events disappear randomly for full calendar module

Question

I am using full calendar module, what happens is sometimes events disappear randomly and sometimes it works fine below is my code

server side code:

@OnEvent(REFRESH_EVENT_NAME)
public Object onRefresh() {
    String start = request.getParameter("start_time");
    String end = request.getParameter("end_time");
    List<Messages> messages = getMessagesfromEs(start,end);
    return new SuccessAjaxResponseBuilder().addJsonParameter(
            new JSONObject().put("events", getEvents(messages))).build();
}

client side code:

let calendar = new FullCalendar.Calendar(calendarEl, {
    plugins: ["dayGrid", "timeGrid", "interaction"],
    defaultView: "dayGridMonth",
    aspectRatio: aspectRatio,
    events: function (info, successCallback, failureCallback) {
        $element.trigger(params.events.refreshEvent, {
            start_time: info.start.valueOf(),
            end_time: info.end.valueOf(),
            success: function (data) {
                var eventsObject = data.parameters[0].events;
                return successCallback(eventsObject);
            },
        });
    },
    eventLimit: limit,
    locale: params.misc.locale,
    now: convertDateToTimeZone().toISOString(),
    dir: params.misc.isRTL ? "rtl" : "ltr",
    fixedWeekCount: false,
    displayEventTime: true,
    height: "auto",
    header: {
        left: headerLeft,
        center: headerCenter,
        right: headerRight,
    },
    footer: footer,
    Format: {
        hour: "numeric",
        minute: "2-digit",
        meridiem: "short",
    }

The error says unexpected error occured in console. it is mostly due to refresh event. what am i missing here