Kendo Multiselect filter does not show

Question

I have a pretty basic MultiSelect, but the filter textbox (so you can type) does not even render.

@(Html.Kendo().MultiSelectFor(x => x.ParentAction.CustomFields)
    .Placeholder("Search...")
    .BindTo(Model.ParentAction.CustomFields)
    .DataTextField("Name")
    .DataValueField("CustomFieldId")
    .Filter(FilterType.Contains)
    .HtmlAttributes(new { @class = "form-control"})
    .Height(325))

I also have a working DropDownList that uses server filtering:

@(Html.Kendo().DropDownListFor(x => x.ParentAction.CampaignId)
    .OptionLabel("Select an existing campaign")
    .DataSource(s =>
    {
        s.Custom()
            .Type("aspnetmvc-ajax")
            .ServerFiltering(true)
            .Transport(t => t.Read(r =>
            {
                r.Action("GetCampaignsByActionType", "Campaign");
            })
        );
    })
    .DataTextField("Name")
    .DataValueField("CampaignId")
    .Enable(false)
    .AutoBind(false)
    .Filter(FilterType.Contains)
    .CascadeFrom("ParentAction_CampaignType")
    .Height(325)
    .HtmlAttributes(new { @class = "form-control", style = "border:none;" })
    .Deferred(!Request.IsAjaxRequest()))

That I converted to a multiselect which still does not render a textbox that I can type in:

@(Html.Kendo().MultiSelectFor(x => x.ParentAction.CampaignId)
    //.OptionLabel("Select an existing campaign")
    .DataSource(s =>
    {
        s.Custom()
            .Type("aspnetmvc-ajax")
            .ServerFiltering(true)
            .Transport(t => t.Read(r =>
            {
                r.Action("GetCampaignsByActionType", "Campaign");
            })
        );
    })
    .DataTextField("Name")
    .DataValueField("CampaignId")
    .Enable(false)
    .AutoBind(false)
    .Filter(FilterType.Contains)
    //.CascadeFrom("ParentAction_CampaignType")
    .Height(325)
    .HtmlAttributes(new { @class = "form-control", style = "border:none;" })
    .Deferred(!Request.IsAjaxRequest()))

I cannot seem to find anything on this specific behavior when searching the web. So what am I doing incorrectly?