Memory leak

[stubbornbe]

System layout
OS: Debian 13.5
Webserver: Apache 2.4.67
Php: 8.4
PhpSpreadsheet: 5.8.0

Problem
PhpSpreadsheet doesn't release any memory after script end.

Minimalistic script showing issue

$reader = new \PhpOffice\PhpSpreadsheet\Reader\Xlsx();
$reader->setReadDataOnly(true);
$reader->setLoadSheetsOnly(["MyTab", "MyTab"]);
$worksheet = $reader->load($_FILES['excel']['tmp_name']);

+++Here I should execute some code, but even when I do absolutely nothing here, I already get this behavior+++

$worksheet->disconnectWorksheets();
unset($worksheet);
unset($reader);
\PhpOffice\PhpSpreadsheet\Settings::getCache()->clear();
gc_collect_cycles();

Every time I run this script to read an xlsx file, system memory grows significantly. (No problem with that.)
When the above script ends, no memory is released at all.
After a couple of runs all system memory is in use and the entire OS locks on heavy swapping.
I can't find a way to force a memory release other than an Apache restart.
What could be locking? I'm only opening and reading the document.