Return value from using statement with data loss and inaccurate notation?

Question

C++ / C#

Why is the file being written with data loss when exiting or returning a value from a using statement.
For example, here is the archiving code, in 1 case everything is OK and for example the compressed archive is 100kb and can be restored back, and in the second case the compressed archive is 80 and damaged.
Why is that.

public MemoryStream GZipCompress(MemoryStream memoryStream)
        {
            var newStream = new MemoryStream((int)memoryStream.Length / 2); //set to estimate of compression ratio

            using (GZipStream compress = new GZipStream(newStream, CompressionMode.Compress,true))
            {
                memoryStream.CopyTo(compress);
            }  
// Вот так все отлично
            newStream.Position = 0;
            return newStream;
        }

Here there is a loss of data and the data will not be restored, return from under using

public MemoryStream GZipCompress(MemoryStream memoryStream)
        {
            var newStream = new MemoryStream((int)memoryStream.Length / 2); 

            using (GZipStream compress = new GZipStream(newStream, CompressionMode.Compress,true))
            {
                memoryStream.CopyTo(compress);

      // а вот так сжатие  ошибочное
                newStream.Position = 0;
                return newStream;
            }
        }

Answer 1

using expands into a block like this.

var reader = new StringReader(manyLines);
    try {
        string? item;
        do {
            item = reader.ReadLine();
            Console.WriteLine(item);
        } while(item != null);
    } finally
    {
        reader?.Dispose();
    }

So in the second case, everything is already destroyed. And an error occurs