sort of like nginx captures it and removes it from the url as it proxies it.
That can't be the case, because NodeBB doesn't respond without the /forums. For example, if I put the IP and port into my address bar (http://192.168.0.10:4567/), I get a connection refused error, but http://192.168.0.10:4567/forums responds. Edit: This isn't to say that the sub-folder installation couldn't be the cause of the problem - just that NginX seems to be passing the URL correctly.
I'm kind of wondering if this has something to do with SSL.