Hi Darren,
Thank you for reporting this issue! I updated your Telerik points for it.
Until we fix it, you can use the following _onElementFocus function override as a temporary solution for the issue:
        <script>
            var flag = false;
            (function () {
                $(document).on('mousedown', '.RadPanelBar .rpItem', function (e) {
                    $(this).focus();
                    flag = true;
                })
            })();
            _onElementFocus = Telerik.Web.UI.RadPanelBar.prototype._onElementFocus;
            Telerik.Web.UI.RadPanelBar.prototype._onElementFocus = function (e) {
                if (!flag) {
                    _onElementFocus.call(this, e);
                }
            }
        </script>
Regards,
 
Rumen
 Progress Telerik