Unfortunately the enums in question are not defined by me.
Actually, one of them is Microsoft.VisualBasic.MsgBoxStyle in the
Microsoft.VisualBasic assembly.
As you can see below, there are three zero values meaning OkOnly and
DefaultButton1 are appended when calling ToString on any non-zero value.

[Flags]
public enum MsgBoxStyle
{
   ApplicationModal = 0,
   OkOnly = 0,
   DefaultButton1 = 0,
   OkCancel = 1,
   AbortRetryIgnore = 2,
   YesNoCancel = 3,
   YesNo = 4,
   RetryCancel = 5,
   Critical = 0x10,
   Question = 0x20,
   Exclamation = 0x30,
   Information = 0x40,
   DefaultButton2 = 0x100,
   DefaultButton3 = 0x200,
   SystemModal = 0x1000,
   MsgBoxHelp = 0x4000,
   MsgBoxSetForeground = 0x10000,
   MsgBoxRight = 0x80000,
   MsgBoxRtlReading = 0x100000,
}

For the time being, I've found a workaround using a TypeConverter and some
simple string manipulation:

static void FlagsEnumTest(Type type, object value)
{
   TypeConverter converter = TypeDescriptor.GetConverter(type);
   Enum[] items = (Enum[])converter.ConvertTo(value, typeof(Enum[]));
   foreach (Enum item in items)
   {
       if (Enum.IsDefined(type, item))
       {
           string itemName = item.ToString();
           int pos = itemName.LastIndexOf(", ");
           if (pos >= 0)
           {
               itemName = itemName.Substring(pos + 2);
           }
           Console.WriteLine(itemName);
       }
       else
       {
           //contains undefined parts
       }
   }
}