configuration.php unwritable when permission is 0644 but writable when 0664

Question

My Joomla is 3.9.4. When I tried to make some global configuration change in the backend, I got this error:

Could not write to the configuration file.

I looked at the file system, configuration.php has the right owner and is set to 0644, and I can make changes to it via FTP.

I then tried setting the permission to 0777, it is writable; then I tried 0666 and 0664 and it is also good.

Only when I set it to 0644 do I get the above error. I've never encountered this kind of problem before. I suspect it is something wrong with my server(I host the website on a VPS on linode), but I couldn't figure out.

What could possibly be wrong?

UPDATE I found some more clues. This website was duplicated from another server where the website ran well. When I look both site's FPA, I found that on the problematic server, the permission section is like

While on the other server, it is

I don't understand how come one is writable whereas the other is not while both permissions are 0755.

Answer 1

The answer is here, need to set php running mode to FastCGI: https://docs.joomla.org/Why_can%27t_you_install_any_extensions%3F#File_ownership_advice_from_ianmac