The issue is replicated in R3 2021 only. 

<telerik:RadTabStrip runat="server"  ID="RadTabStrip1" >
    <Tabs>
        <telerik:RadTab Text="Tab 1">
            <Tabs>
                <telerik:RadTab Text="fail"></telerik:RadTab>
            </Tabs>
        </telerik:RadTab>
        <telerik:RadTab Text="Tab 2"></telerik:RadTab>
        <telerik:RadTab Text="Tab 3"></telerik:RadTab>
    </Tabs>
</telerik:RadTabStrip>

Solution from Admin:

Place the load/script below somewhere under the ScriptManager. It can be added in the MasterPage if needed:

<script>
    Sys.Application.add_init(function fixTabStrip() {
        if (Telerik && Telerik.Web && Telerik.Web.UI && Telerik.Web.UI.RadTabStrip) {
            Telerik.Web.UI.RadTabStrip.prototype._childInserted = function (index, tab, owner) {
                this._allTabs = null;

                if (this._cachedSelectedTab) {
                    owner._setSelectedIndex(this._cachedSelectedTab.get_index());
                    this._cachedSelectedTab = null;
                }

                if (tab._cachedSelected) {
                    owner._setSelectedIndex(index);
                    tab._cachedSelected = false;
                }

                Telerik.Web.UI.RadTabStrip.callBaseMethod(this, "_childInserted", [index, tab, owner]);

                if (tab.get_isBreak()) {
                    var breakElement = document.createElement("li");
                    breakElement.className = "rtsBreak";
                    owner.get_childListElement().insertBefore(breakElement, tab._element.nextSibling);
                }

                //if (tab.get_parent() && tab.get_parent() instanceof Telerik.Web.UI.RadTab && tab.get_parent().get_selected()) {
                //    $telerik.$(tab.get_parent().get_levelElement()).removeClass("rtsHidden");
                //}
            }
        }
    });
</script>

No direct way to select the prev/next month and year buttons - While the calendar is open and focused, note that there is no way to focus and use the month and year changing buttons in the calendar header

Related to https://feedback.telerik.com/aspnet-ajax/1481017-jaws-and-keyboard-support-improvements-in-raddatepicker 

JavaScript error is thrown then multiple RadNotification instances with enabled audio are shown.

Error message:

[Intervention] Blocked attempt to create a WebMediaPlayer as there are too many WebMediaPlayers already in existence. See crbug.com/1144736#c27

We are having a touchscreen problem with the RadSearchBox, specifically on the MicroSoft surface touchscreen. I have tested this out on mobile phone and other tablets, but the problem occurs on the Surface.

The issue is that you cannot change the SearchContext, because it never seems to recognize the touch event.

You can touch the SearchContext to initiate the drop-down event, but then it is impossible to select an item from the list.

There are no other problems with Telerik controls on the Surface (the touch events on RadDropDown, RadTabStrip, RadComboBox, RadMenu, etc.) all seem to be working fine.

"Loading..." text in Combobox with Virtual Scrolling is visible on first set of items loading.
It shows first time "Loading..." text. on scrolling it loads items from PageMethods/WebService but "Loading..." text it not visible i.e. because it is first <LI> item so it is not visible.

Instead of first <LI> item, it  should be at position where it always display while loading more items.

same issue is also in telerik demo website : http://demos.telerik.com/aspnet-ajax/combobox/examples/populatingwithdata/autocompletesql/defaultcs.aspx

Under heavy load in a multithreaded environment, a concurrency issue can occur, related to System.Collections.Generic.Dictionary`2.Insert

This might lead to High CPU usage and might require an IIS restart.

Workaround:

• Use CDN for Telerik styles and scripts
  • https://docs.telerik.com/devtools/aspnet-ajax/controls/scriptmanager/cdn-support/custom-cdn-provider 
  • https://docs.telerik.com/devtools/aspnet-ajax/controls/scriptmanager/cdn-support/overview