I see the problem now.
My ISP is using carrier NAT in this neighbourhood (simply, one public IP corresponds to many private client IP). I overlooked this fact during my troubleshooting as I incorrectly assumed that I was assigned a public IP by my ISP.
So it’s likely that some people in my neighbourhood using the same ISP was potentially spamming (or more likely, tried to legitimately sign up for forum) that cause Discourse to flag this IP.
I would admit, blacklisting IP address is a cruel & effective approach to stop spamming. But in a world where IPv4 addresses has already exhausted, it’s a less-elegant solution.
In the end, to quote Wikipedia:
In cases of banning traffic based on IP addresses, the system might block the traffic of a spamming user by banning the user’s IP address. If that user happens to be behind carrier-grade NAT, other users sharing the same public address with the spammer will be mistakenly blocked. This can create serious problems for forum and wiki administrators attempting to address disruptive actions from a single user sharing an IP address with legitimate users.