Finally found out why the Apache httpd service would fail at boot: this server is dual-stack, and the web server's service would try to start before the IPv6 part of the network connection would be established.

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httpd.service:
* Added network-online.target to After=, didn't work;
* Added ExecStartPre=/bin/sleep 5 worked, but that just makes httpd delay start for 5 seconds...
* ...so Restart=on-failure, RestartSec=1s, and StartLimitInterval=1min were to be added instead as a workaround.