ポストバックイベント前後でHTTPクエリ文字列が変わってしまう

Partial Public Class _Default
    Inherits System.Web.UI.Page

    Protected Overrides Sub Render(ByVal writer As System.Web.UI.HtmlTextWriter)
        MyBase.Render(New ActionAttributeFixedHtmlTextWriter(writer))
    End Sub

End Class

Public Class ActionAttributeFixedHtmlTextWriter
    Inherits HtmlTextWriter
    Private uencoderegex As Regex

    Sub New(ByVal original As HtmlTextWriter)
        MyBase.New(original.InnerWriter)
        uencoderegex = New Regex("%u[0-9a-f]{4}", RegexOptions.Compiled Or RegexOptions.IgnoreCase)
    End Sub

    Public Overrides Sub WriteAttribute(ByVal name As String, ByVal value As String, ByVal fEncode As Boolean)
        If name.ToLower = "action" Then
            value = uencoderegex.Replace(value, New MatchEvaluator(AddressOf UEncodeEvaluator))
        End If
        MyBase.WriteAttribute(name, value, fEncode)
    End Sub

    Private Function UEncodeEvaluator(ByVal match As Match) As String
        Return HttpUtility.UrlEncode(HttpUtility.UrlDecode(match.Value), System.Text.Encoding.UTF8)
    End Function

End Class