What web server (Apache, nginx, etc.) and how is PHP running (like via module or FPM)?

And are you sure *execution* stops or can you simply not see the output? Try something like touch()ing a file.

nginx 1.12.2
php-fpm 7.1.12

I've changed the test script in accordance with your advice to the following:
----
header('content-length: 0', false, null);
touch('/var/www/html/'.time());
die('done');
----

Neither the file was created nor the "done" was displayed.

I've rebuilt my container to use php7.2.1 and it's exactly the same.

cause and effect

pretty sure beause of the empty content-length the connection to the fcgi-worker is closed and so as result the script terminated - either use output-buffering or user_abort_ignore(true) and that would be impossible to happen beause with output buffering anything starts to hits the cient *after* the whole script has finished and most applications use ob_stzart() anyways to have the capability send headers at any point in time

http://php.net/manual/de/function.ignore-user-abort.php
http://php.net/manual/de/function.ob-start.php

Were permissions on /var/www/html set to allow the new file? Try in /tmp instead.
And do you have the gzip module installed and enabled in nginx? Try without.

Modified the script again to be:
----
touch('/tmp/1');
header('content-length:0',false, null);
touch('/tmp/2');
die('done');
----

Both 1 and 2 exist, no "done" is displayed, so it looks like it does not terminate...

Can you check that gzip thing with nginx? Though I haven't seen anything to explicitly state it cuts off content, I know it will respect a Content-Length to some degree.

No gzip enabled.