I believe that was by design. You are ignoring the messages only from that person. That means replies others make are not subject to the same ignore restriction.
A staff member can confirm, but my guess based on the fact there is already a message showing the OP is ignored is that was intentional. As a result, it would be working as intended. That if you want to suggest other logic, it would be best proposed in the Feedback forum.
But again, I’m not IPS and don’t speak officially for them… so I’ll slink back in my corner and have a drink. 🙂