To reproduce:
- Set the column like this:
GridViewMaskBoxColumn col = new GridViewMaskBoxColumn();
col.Mask = "&&&&&&&&&&";
col.MaskType = MaskType.Standard;
col.FieldName = "Name";
col.TextMaskFormat = MaskFormat.IncludeLiterals;

- Type two words and press enter.

Workaround:
public class MyRadMaskedEditBoxEditor : RadMaskedEditBoxEditor
{
    public override object Value
    {
        get
        {
            if (this.MaskTextBox.Mask == "my mask")
            {
                return this.MaskTextBox.Value;
            }
            return base.Value;
        }
        set
        {
            base.Value = value;
        }
    }
}