Since you're assuming the property are simple properties, we can simply treat a property of json as a field in csv.

To make the code clear and clean, I define a Row as SortedDictionary<string,string> :

using Row =SortedDictionary<string,string>; 

I also write a helper class to export json to csv.

public class JsonToCsvExporter{

    public JsonToCsvExporter(string json,string sep=","){
        this._json = json;
        this.Sep = sep;
        this.Rows = new List<Row>();
        this.Headers = new List<string>();
        this.Initialize(json);
    }

    private string _json ;

    public IList<Row> Rows{get;set;}
    public IList<string> Headers { get; set; }
    public string Sep {get;set;}=",";

    private void Initialize(string json){
        var o = JArray.Parse(json);
        this.BuildRows(o, null);
        this.Headers = this.Rows.FirstOrDefault().Keys.ToList();
        this.NormailizeRows();
    }
    private void BuildRows(IEnumerable<JToken> tokens, Row row){
        if(row == null){ row = new Row(); }
        foreach( var token in tokens){
            if (token.Type == JTokenType.Property)
            {
                JProperty prop = (JProperty)token;
                if (!prop.Value.HasValues){
                    row.Add(prop.Name,prop.Value.ToString());
                }
            }
            // if it is not a `JProperty`, they shoud have children,
            //     that means it shoud be treated as a brand new line 
            else if (token.HasValues){
                var _row = new Row();
                BuildRows(token.Children(),_row);
            }
        }
        // if current row has fields, add this row
        if (row.Count>0) {
            this.Rows.Add(row);
        }
    }

    // add null for unspecified values
    private void NormailizeRows() {
        foreach (var row in Rows) {
            foreach (var header in Headers) {
                if (!row.ContainsKey(header)) {
                    row.Add(header,null);
                }
            }
        }
    }

    private async Task ForEach<T>(IEnumerable<T> items,Func<T,Task> funcForFirst,Func<T,Task> funcForEach ){
        if(funcForFirst== null ){ throw new ArgumentNullException(nameof(funcForFirst));}
        if(funcForEach== null ){ throw new ArgumentNullException(nameof(funcForEach));}

        var iter = items.GetEnumerator();
        var flag= iter?.MoveNext();
        if(flag==false){ throw new Exception("items MUST have at least one element");}

        await funcForFirst(iter.Current);

        while(iter.MoveNext()!= false){
            await funcForEach(iter.Current);
        }
    }

    public async Task ExportHeader(StreamWriter writer){
        await this.ForEach(this.Headers,
            async header=>{
                await writer.WriteAsync(header);
            },
            async header=>{
                await writer.WriteAsync(this.Sep);
                await writer.WriteAsync(header);
            }
        );
        await writer.WriteLineAsync();
    }

    public async Task ExportBody(StreamWriter writer)
    {
        foreach (var row in this.Rows) {
            await this.ForEach(row,
                async f=>{
                    await writer.WriteAsync(f.Value);
                },
                async f=>{
                    await writer.WriteAsync(this.Sep);
                    await writer.WriteAsync(f.Value);
                }
            );
            await writer.WriteLineAsync();
        }
    }

}

How to Use and Test Case

static void Main(string[] args)
{
    var json =@"[{
        'F1': 'hello1',
        'F2': 'world1',
        'F3': 'foo1',
        'F4': 'bar2',
    },{
        'F1': 'Hello2',
        'F4': 'Bar2',
    },{
        'F1': 'Hello3',
        'F2': 'World3',
        'F3': null,
        'F4': 'Bar3',
    }]";
    var fs= new FileStream("xxxx.csv",FileMode.OpenOrCreate);
    using(var writer = new StreamWriter(fs)){
        var exporter= new JsonToCsvExporter(json);
        exporter.ExportHeader(writer).Wait();
        exporter.ExportBody(writer).Wait();
        fs.Flush();
    }
}