Bootstrap modal losing textboxvalue after postback

I have some strange behavior with a bootstrap modal, the modal contains a textbox and an asp:button

When I click the button and inspect the value of the textbox in VB code behind - results in an empty string

If I remove the first and last lines from the code below, I can successfully read the value in the textbox from code-behind

Here is the modal markup:

 <div class="modal inmodal" id="modalUpdateDialog" tabindex="-1" role="dialog" aria-hidden="true">
        <asp:UpdatePanel ID="udpUpdateDialog" runat="server" UpdateMode="Conditional">
            <ContentTemplate>
                <div class="modal-dialog">
                    <div class="modal-content animated bounceInDown">
                        <div class="modal-header">
                            <button type="button" class="close" data-dismiss="modal"><span aria-hidden="true">×</span><span class="sr-only">Close</span></button>
                            <h4 class="modal-title">Update File</h4>
                            <small class="font-bold">Please enter your new description</small>
                        </div>
                        <div class="modal-body form-horizontal" style="height: auto; overflow: auto;">
                            <div class="form-group">
                                <label class="col-sm-2 control-label">Description</label>
                                <div class="col-sm-10">
                                    <asp:TextBox ID="txtNewDescription" runat="server" CssClass="form-control" />
                                    <asp:HiddenField ID="hdnRowID" runat="server" />
                                </div>
                            </div>
                        </div>
                        <div class="modal-footer">
                            <asp:Button ID="btnUpdateFile" runat="server" Text="Update File" class="btn btn-primary" UseSubmitBehavior="false" />
                        </div>
                    </div>
                </div>
            </ContentTemplate>
        </asp:UpdatePanel>
    </div>

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