Multiselect combo not returning values

Hello, 

I have followed this article to create a multiselect combo box. 

https://asiablog.acumatica.com/2016/03/multiselect-combo-box.html

Everything operates in the way it is shown in the article, values are displayed in the combo box, I can select 2 values and those values are saved to the database. 

However, when I leave the current record and then return to the record, the combo box value is empty. The values are in the database but they don’t appear on the combo box. 

Here is my DAC

[PXDBString(255, IsUnicode = true)]
[PXUIField(DisplayName = "Brands")]
[PXStringList(new string[] { "D", "C", "T", "I" }, new string[] { "Development", "Consulting", "Training", "Implementation" }, MultiSelect = true)]
public virtual string UsrBrands { get; set; }
public abstract class usrBrands : PX.Data.BQL.BqlString.Field<usrBrands> { }

..and the ASPX

<px:PXDropDown runat="server" ID="CstPXSelector334" Size="M" DataField="usrBrands" AllowMultiSelect="True" />

I’m not sure what else is required, can anyone help please?