I have changed it to use a timer. It’s much cleaner, as it does exactly what we want without altering other service behavior.
For anyone willing to apply the fix manually, create a file /etc/systemd/system/nginx.timer
with the following content:
[Unit]
Description=Start Nginx after boot
PartOf=nginx.service
[Timer]
OnActiveSec=10
Unit=nginx.service
[Install]
WantedBy=multi-user.target
After reboot Nginx should always be running!