using (SqlDataReader reader = await command.ExecuteReaderAsync(CommandBehavior.SequentialAccess))
{
    if (await reader.ReadAsync() && !await reader.IsDBNullAsync(1))
    {
        using var buffer = new MemoryStream(originalData.Length);

        // Doesn't have any bytes to read
        using (Stream stream = await reader.GetFieldValueAsync<Stream>(1))
        {
            await stream.CopyToAsync(buffer);
            outputData = buffer.ToArray();
        }

        // Works
        //using (Stream stream = reader.GetStream(1))
        //{
        //    await stream.CopyToAsync(buffer);
        //    outputData = buffer.ToArray();
        //}
    }
}

Debug.Assert(originalData.SequenceEqual(outputData));