NEST is returning filebeat @timestamp as null always

Elastic Stack Elasticsearch
1

when retrieving documents with NEST with this code:

        var result = elk.Search<FileBeatDto>(x => x
            .Index("filebeat-*")                
            .AllIndices()
            .Type(type)                               
            .Query(q => 
                q.Match(qs => qs.Field("fields.asset_tag").Query(asset_tag))  &&
                q.Match(qs => qs.Field("message").Query(filter))
                )
            .Take(lines)
            );

I get documents but the timestamp is always null.

This is FilebeatDto:

public class FileBeatDto
{
    public DateTime timestamp { get; set; }
    public string datetime { get; set; }
    public DateTime received_at { get; set; }
    public int offset { get; set; }
    public string version { get; set; }
    public string input_type { get; set; }
    public int count { get; set; }
    public Beat beat { get; set; }
    public string host { get; set; }
    public string source { get; set; }
    public string message { get; set; }
    public string type { get; set; }
    public object fields { get; set; }
    public List<string> tags { get; set; }
}

Any idea why timestamp is null? I tried with @timestamp and timestamp.