realpath cache not working

Hi,
On my VPS with Plesk 12.5.3 realpath cache only lives for a few seconds.
Apache, Nginx witj PHP-FPM, http2 enabled, PHP 7. 4GB of memory, 2 core processor.
Opcache on, openbase_dir none.
Settings:
realpath_cache_size = 1024k
realpath_cache_ttl = "3600"

Hoster doesn't know.
Any idea how to solve this? Is this Plesk related?

Thanks.