Before binding your server to 0.0.0.0, please be clear about the security implications of these changes: Your server is publicly exposed to any or all IPs on the entire web. Be sure to enable authentication on your server!
You can't access your machine once you bind it to 127.0.0.1 on EC2. That's not a bug, it's reasoned by the network interface bindings.
127.0.0.1 can only bind to the loopback interface (so you'll only be able to access it locally), while 0.0.0.0 will bind it to any or all network interfaces that are accessible.
That's why you'll be able to access your MongoDB on EC2 once you bind it to 0.0.0.0(as it's available through the web now) and not via 127.0.0.1.
For local servers (like a WAMP or a local MongoDB server) that won't look different to you, but for that case you should also think that binding to 0.0.0.0 for local servers might build them available overall network interfaces (so it'd be public for somebody who knows your IP, if there's no firewall!)