I have migrated a few WordPress websites from a shared hosting to a VPS. WordPress has an option of redirecting PHP errors from the regular error.log to its own debug.log file.
Now I have a problem: When PHP generates a Notice, Warning, Error, it is only written to the debug.log file at even hours o'clock. 2:00, 4:00, 6:00, 8:00 ... So if there is a PHP Warning at 16:01, I have to wait 1 hour 59 minutes until it is written to the log at 18:00. This is NOT a WordPress function - the websites were duplicated from another host and there was no such issue. Nor have I seen anything like this ever before.
I looked at apache conf files, enabled mods, site conf files, php conf file, found nothing suspicious. There was nothing in the crontab to run every 2 hours.
Is there any PHP or Apache functionality that would facilitate file write buffering with scheduled writes like this? Has anyone encountered this? The VPS provider has no idea.