Comment 1 for bug 1823724

You shouldn't modify the /lib/systemd/service/apache2.service file directly, but use an override instead.

For example, you can do "sudo systemctl edit apache2.service" and add your modifications in the editor that will open. This will create an override file in /etc/systemd/system/ for apache2, and a package upgrade won't change that.

"Example 2" at https://www.freedesktop.org/software/systemd/man/systemd.unit.html actually mentions your case, of setting PrivateTmp

I'll mark this bug as invalid, but if you think that's not the case, feel free to reopen it with a comment and reasoning.