A workaround is to place the rwLoading class in the proper place by using the events of the control, for example:

	<telerik:RadWindowManager ID="RadWindowManager1" runat="server" ShowContentDuringLoad="false" 
		OnClientShow="applyProperLoadingSign" OnClientPageLoad="removeLoadingSignClass">
	</telerik:RadWindowManager>
		<script type="text/javascript">
			function applyProperLoadingSign(sender, args) {
				if (!sender.get_showContentDuringLoad() && $telerik.isMobileSafari) {
					$telerik.$(".rwIframeWrapperIOS", sender.get_popupElement()).removeClass("rwLoading");
					$telerik.$(".rwWindowContent.rwExternalContent", sender.get_popupElement()).addClass("rwLoading");
				}
			}

			function removeLoadingSignClass(sender, args) {
				if ($telerik.isMobileSafari) {
					$telerik.$(".rwLoading", sender.get_popupElement()).removeClass("rwLoading");
				}
			}
                       </script>