aspxgridview search filter problem 'doesn't contain'

Question

Hi I'm working on an aspx application.

In this I have an aspxgridview with filters applied to the gray, these in addition to having a textbox also have a button that allows the selection of the type of search.

these support a property called "FilterExpression" and so far so good, when I perform an action on my page, I keep the value of "FilterExpression" so in the event of an error I can allow the grid to remain filtered without losing the values, and it works well for all filters, except for 'doesn't contain'.

When I perform an action I have an error handled and therefore I return to my grid, the datasource of the grid remains filtered as I pass the value I have kept of FilterExpression, but despite this the typing textbox for the search becomes white, and I don't understand why it just do it for that type of filter.

Code of my grid:

 <dx:ASPxGridView ID="wdtgRagPolizzaEffettiva" runat="server" CssClass="searchResultsGrid" Width="100%"
 KeyFieldName="UNIV_POLIZZA;UNIV_MOVIMENTO"
 OnCustomJSProperties="wdtgRagPolizzaEffettiva_CustomJSProperties"
 ClientInstanceName="gvwRagPolizzaEffettiva"
 OnDataBinding="wdtgRagPolizzaEffettiva_DataBinding"
 OnRowCommand="wdtgRagPolizza_RowCommand" EnableCallBacks="false"
 SettingsBehavior-AllowSelectByRowClick="true" SettingsBehavior-AllowSelectSingleRowOnly="false"
 OnSelectionChanged="wdtgRagPolizzaEffettiva_SelectionChanged1"
 SettingsBehavior-AllowFocusedRow="false"
 OnBeforeGetCallbackResult="wdtgRagPolizzaEffettiva_BeforeGetCallbackResult" OnBeforeHeaderFilterFillItems="wdtgRagPolizzaEffettiva_BeforeHeaderFilterFillItems"
 OnFillContextMenuItems="Grid_FillContextMenuItems" OnDetailRowExpandedChanged="wdtgRagPolizzaEffettiva_DetailRowExpandedChanged">
 <ClientSideEvents SelectionChanged="OnGridSelectionChanged" />
 <Columns>
     <dx:GridViewCommandColumn ShowSelectCheckbox="true" Visible="true" Width="30px">
         <HeaderTemplate>
             <dx:ASPxCheckBox ID="cbSelectAll" runat="server" ClientInstanceName="cbSelectAll"
                 ToolTip="<%$Resources: GridColumns, cbAllToolTip %>" OnInit="cbSelectAll_Init">
                 <ClientSideEvents CheckedChanged="OnAllCheckedChanged" />
             </dx:ASPxCheckBox>
         </HeaderTemplate>
     </dx:GridViewCommandColumn>

     <dx:GridViewDataTextColumn FieldName="DEFI_NUMPOLIZZA" Visible="true" Width="80px"
         Caption="<%$Resources:GridColumns, DEFI_NUM_POLIZZA%>">
         <DataItemTemplate>
             <asp:LinkButton ID="wlnkDett" runat="server"
                 CommandName="OpenUrl"
                 Visible="true"
                 CommandArgument="openNewTab"
                 Text='<%# Eval("DEFI_NUMPOLIZZA") %>'>                                                            
             </asp:LinkButton>
         </DataItemTemplate>
     </dx:GridViewDataTextColumn>

     <dx:GridViewDataTextColumn FieldName="DEFI_RIF_CLIENTE" Visible="true" Width="80px"
         Caption="<%$Resources:GridColumns, DEFI_RIF_CLIENTE%>" />

     <dx:GridViewDataTextColumn FieldName="UNIV_TIPO_POL" Visible="false" Width="80px"
         Caption="<%$Resources:GridColumns, UNIV_TIPO_POL%>" />

     <dx:GridViewDataTextColumn FieldName="BOOL_STORNATO" Visible="false" Width="80px"
         Caption="<%$Resources:GridColumns, DEFI_RIF_CLIENTE%>" />

     <dx:GridViewDataTextColumn FieldName="VALIDITA" Visible="false"
         Caption="<%$Resources:GridColumns, VALIDITA%>" />

     <dx:GridViewDataColumn FieldName="DATA_EFFETTO_POL" Visible="true" Width="80px"
         Caption="<%$Resources:GridColumns, DATA_EFFETTO_POL%>" />

     <dx:GridViewDataColumn FieldName="DATA_SCAD_POL" Visible="true" Width="80px"
         Caption="<%$Resources:GridColumns, DATA_SCAD_POL%>" />

     <dx:GridViewDataTextColumn FieldName="CONTRAENTE" Visible="true" Width="150px"
         Caption="<%$Resources:GridColumns, CONTRAENTE%>" />
     <dx:GridViewDataTextColumn FieldName="ASSICURATO" Visible="true" Width="150px"
         Caption="<%$Resources:Servizi, StatistichePersonalizzate_Text_Assicurato%>" />

     <dx:GridViewDataTextColumn FieldName="OGGETTO_ASSICURATO" Visible="true" Width="120px"
         Caption="<%$Resources:GridColumns, DEFI_NOMINATIVO3%>" />

     <dx:GridViewDataTextColumn FieldName="DEFI_MOVIMENTO" Visible="true" Width="125px"
         Caption="<%$Resources:GridColumns, DEFI_MOVIMENTO%>" />

     <dx:GridViewDataTextColumn FieldName="STATO_MOV" Visible="true" Width="125px"
         Caption="<%$Resources:GridColumns, STATO%>" />

     <dx:GridViewDataTextColumn FieldName="BOOL_PROVVISORIO" Visible="false" Width="125px"
         Caption="<%$Resources:GridColumns, STATO%>" />

The problem is in the MOV STATUS column. I update the filters using a session variable 'sFilterExpressionPolizzaEffetiva' like this:

  sFilterExpressionPolizzaEffettiva = wdtgRagPolizzaEffettiva.FilterExpression;
  wdtgRagPolizzaEffettiva.DataSource = Session["RagPolizzaEffettiva"];