@stephenw10 I tried again today. This time I used my latest config backup in version 2.6, deleted the said rule and its duplicates (size 398KB), and then rebooted. It booted up properly with config size 399KB. I initially got no internet and later found my LAN interface rule, which allows any sources/ports was missing. Adding the rule back and the internet was back. I tried changing a few configurations and the config file has not grown like before.
I'll keep monitoring whether there will be any other side effects.
Thanks.
update on Oct 2, 2023: The problem has been permanently fixed, though I don't know how such Openvpn wizard creation xml section was there.