Finding Document Names

[faurepa]

Hello,

I have come across something odd. I am looking for DocumentNumber, the following code works most of time time.
var myList = context.Web.Lists.GetByTitle("Current", p => p.Title,
p => p.All,
p => p.Fields.QueryProperties(p => p.InternalName,
p => p.FieldTypeKind,
p => p.TypeAsString,
p => p.Title));

                await foreach (var listItem in myList.Items)
                {
                        // Do something with the list item
                    if (listItem["Campus"].ToString() == "Cambridge")
                    {
                        if (listItem.Values.GetValueOrDefault("N_x002f_DocumentNumber") == null)
                        {
                            Console.WriteLine("I got here");
                            //IFile testDocument = context.Web.GetFileById(listItem.Values.GetValueOrDefault("ContentTypeId").ToString();
                        }
                        Console.WriteLine($"{Intcounter} : Location:{listItem.Values.GetValueOrDefault("Campus")} : ");
                        Console.WriteLine($"{Intcounter} : Filename:{listItem.Values.GetValueOrDefault("N_x002f_DocumentNumber")} : ");
                        Console.WriteLine($"{Intcounter} : Revision:{listItem.Values.GetValueOrDefault("Revision")} : ");
                        Console.WriteLine($"{Intcounter} : ReleaseDate:{listItem.Values.GetValueOrDefault("ReleaseDate")} : ");
                        Console.WriteLine($"{Intcounter} : Title:{listItem.Title} : ");
                        Console.WriteLine($"{Intcounter} : Id:{listItem.Id} : ");
                        Console.WriteLine($" ");
                    }
                }

what I am finding is that in some cases, "N_x002f_DocumentNumber" is actually null. there is a Campus, Revision, Title, Id.
now when I look at the site online, I can get the document number it is the column called "name".

I was wondering if anyone would have some insight on how to get the name in call cases?
I thought of trying to get the File by Guid, but I can only find the Server URL. When I try that, I get the follwoing error message:
PnP.Core.SharePointRestServiceException: 'SharePoint Rest service exception'

thank you

Philippe