We do something similar. For whatever reason we didn't see it take effect until Logstash was restarted. But we were only looking at one very common IP in our logs.
If you check the documentation there is a config value you can set. Sounds like it does cache a certain amount of IP's. I have no idea how long it stays in there though before being evicted though. Could be a few minutes, could be forever. It is outside of my expertise.
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