tcp6 0 0 :::3515 :::* LISTEN
This shows that Logstash is listening on all interfaces. Have you verified that it's impossible to reach Logstash from any of the networks?
How do I configure logstash to listen to multiple tcp inputs on the same port?
You currently have one tcp input. Make a copy of it but make sure both have unique host options containing the hosts's IP address for each network interface. But again, Logstash already listens on all interfaces so I don't think it'll make a difference.