Found: 5.3.0
Ext.NET Forums' thread: Stateful grid with cell editing - reconfiguration causes the cell editor to be destroyed

GridPanel columns' Editor config doesn't take LazyMode into account when creating its output and always use LazyMode.Instance. In case one needs the columns defined as config objects (LazyMode.Config), for instance to allow using the columns definition to .reconfigure() the grid, the only means is thru CustomConfig passing the editors as server-side instances of the component converted with the .ToConfig() method.

For example in Ext.NET MVC with Razor syntax:

Html.X().Column().Text("Company").DataIndex("company").Flex(1)
    .CustomConfig(c => c.Add(
        new ConfigItem("editor", new Ext.Net.TextField() { AllowBlank = false }.ToConfig(), ParameterMode.Raw))
    ),

So that the column is output as:

{
    "editor": {
        xtype: "textfield",
        validateOnFocusLeave: true,
        allowBlank: false
    },
    flex: 1,
    dataIndex: "company",
    text: "Company"
}

The only currently possible to output the config is instancing the corresponding Ext JS client-side class, which makes it unable to be referenced later e.g. to pass as a parameter to App.GridPanel1.reconfigure(App.GridPanel1.getStore(), App.GridPanel1.getInitialConfig().columns.items). The instance output layout, in contrast with the desired above, is:

{
    flex: 1,
    dataIndex: "company",
    editor: new Ext.grid.CellEditor(Ext.apply({
        field: {
            xtype: "textfield",
            validateOnFocusLeave: true,
            allowBlank: false
        }
    }, {})),
    text: "Company"
}