PHP - GET variables sometimes missing on header redirect (cloudflare related?)

I've a strange problem with header() redirects.
There are three pages involved: upload.php (for choosing the file, POST form), doupload.php (add file to server, write to DB etc.) and target.php (doupload.php redirects to that page after sucessful upload).

My code on doupload.php at the end (the redirecting part):

$id = 5;
header("Location: https://www.mypage.com/target.php?id=".$id."&secondvar=1");
exit();


On normal circumstances it works without problems. But sometimes the connection to my webserver lacks (bad connection) and it needs 5 up to 20 seconds to complete the request.
In this case, all GET variables after 'id=5' are gone.

Example: Should be redirected to

/target.php?id=5&secondvar=1


but it's redirected to

/target.php?id=5


without the rest.
I'm using Cloudflare. Maybe there is a session problem in this cases?
I could add session_start(); to each page, but I don't think that's the problem, because on normal circumstances it works.

I'm very thankful for every help to fix that strange issue.

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