Update1 : there might be some hidden setting that directs to cache tables? (both sites can possibly access the other database independently of settings.php)
Update 2 : running cron clear-up the things too
Update 3 : this happens also with another site completely independent with a different file / database owner (but also with the same code and database, only settings.php are different : database login and cookies domain
The virtual servers are configured in FCGID, PHP 7.4
Critical cross-caching between independent sites on the same physical server
Set up a clone of an existing site
This clone is in its own file system, its own database, but they are owned by the same user
On the new site, go to /admin/config/media/file-system and change the temporary directory
On the existing site, go to the same page : the temporary directory changed to take the same value as the new one !
Don't change nor save this page, go to the database of the existing site and check the variable 'file_temporary_path' : it is correct.
Clear the cache : the original value is back.
You can proceed the other way : changing any setting on one site is reflected in the other site when opening the corresponding page/form
Thanks for help!
Fixed
7.0 ⚰️
I found the problem.
I am using memcache that I suspected to be the culprit. However, disabling Memcache did not solve the issue.
I then realized that disabling Memcache does not actually disable it.
What was missing in my setting.php this line:
$conf['memcache_key_prefix'] = 'something_unique';
Automatically closed - issue fixed for 2 weeks with no activity.