RadDockLayout cannot persists its state if contains more than 9 docks.
Code to reproduce (if one of RadDocks is removed the state will be persisted properly):
<telerik:RadDockLayout runat="server" ID="RadDockLayout1" EnableLayoutPersistence="true" RenderingMode="Lightweight"
LayoutPersistenceRepositoryType="Cookies" LayoutRepositoryID="repID20210406_1">
<telerik:RadDock RenderMode="Lightweight" ID="RadDock1" runat="server"
UniqueName="RadDock1" AutoPostBack="true">
<ContentTemplate>
dock 1
</ContentTemplate>
</telerik:RadDock>
<telerik:RadDock RenderMode="Lightweight" ID="RadDock2" runat="server"
UniqueName="RadDock2" AutoPostBack="true">
<ContentTemplate>
dock 2
</ContentTemplate>
</telerik:RadDock>
<telerik:RadDock RenderMode="Lightweight" ID="RadDock3" runat="server"
UniqueName="RadDock3" AutoPostBack="true">
<ContentTemplate>
dock 3
</ContentTemplate>
</telerik:RadDock>
<telerik:RadDock RenderMode="Lightweight" ID="RadDock4" runat="server"
UniqueName="RadDock4" AutoPostBack="true">
<ContentTemplate>
dock 4
</ContentTemplate>
</telerik:RadDock>
<telerik:RadDock RenderMode="Lightweight" ID="RadDock5" runat="server"
UniqueName="RadDock5" AutoPostBack="true">
<ContentTemplate>
dock 5
</ContentTemplate>
</telerik:RadDock>
<telerik:RadDock RenderMode="Lightweight" ID="RadDock6" runat="server"
UniqueName="RadDock6" AutoPostBack="true">
<ContentTemplate>
dock 6
</ContentTemplate>
</telerik:RadDock>
<telerik:RadDock RenderMode="Lightweight" ID="RadDock7" runat="server"
UniqueName="RadDock7" AutoPostBack="true">
<ContentTemplate>
dock 7
</ContentTemplate>
</telerik:RadDock>
<telerik:RadDock RenderMode="Lightweight" ID="RadDock8" runat="server"
UniqueName="RadDock8" AutoPostBack="true">
<ContentTemplate>
dock 8
</ContentTemplate>
</telerik:RadDock>
<telerik:RadDock RenderMode="Lightweight" ID="RadDock9" runat="server"
UniqueName="RadDock9" AutoPostBack="true">
<ContentTemplate>
dock 9
</ContentTemplate>
</telerik:RadDock>
<%--The state will be persisted of you comment the dock below--%>
<telerik:RadDock RenderMode="Lightweight" ID="RadDock10" runat="server"
UniqueName="RadDock10" AutoPostBack="true">
<ContentTemplate>
dock 10
</ContentTemplate>
</telerik:RadDock>
</telerik:RadDockLayout>