This looks like a problem upstream.

Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc. If the script requires a 
database to demonstrate the issue, please make sure it creates 
all necessary tables, stored procedures etc.

Please avoid embedding huge scripts into the report.

No feedback was provided. The bug is being suspended because
we assume that you are no longer experiencing the problem.
If this is not the case and you are able to provide the
information that was requested earlier, please do so and
change the status of the bug back to "Open". Thank you.

To reproduce this error you can use the following three scripts. index.php and hello.php are placed on an Apache-Webserver. If you call test.php it will load index.php, which returns a content-length of 3 and redirects to hello.php, which uses output-buffering and thus causes Apache to omit the content-length header. The result is that "download_content_length" returned by cURL is 3 instead of NULL or -1.
This might be a bug in libCurl though.

test.php
<?php
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "http://php.local.site/index.php");
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_VERBOSE, 1);
curl_setopt($ch, CURLOPT_HEADER, 1);
$output = curl_exec($ch);
$info = curl_getinfo($ch);
curl_close($ch);
echo "\ndownload_content_length => ".$info['download_content_length']."\n";
?>

index.php
<?php
header('Location: http://php.local.site/hello.php');
echo "...";
?>

hello.php
<?php
ob_start();
echo 'hello world';
ob_flush();
flush();
ob_end_flush();
?>