I couldn’t think of any reason why this would be happening so at this point I would assume it’s some weird setting that is causing the issue.
First make sure you’re not doing 2 separate Standalone sessions. Be sure that you are running a listen server.
Then I would create a new PlayerController class and create a really simple function like what I did in blueprints above, (but do it in C++) and see if you’re able to get the multicast to execute on both server and client.
If it still doesnt work, I would assume it’s something with my project, then i would create a new project, and then try to get a simple function that communicates like it should