Recently in one of the project we faced a situation where we have used socket.io on backend and Android app connects to it. Socket.io was using http protocol for testing. It worked fine in older android versions but faced an issue in Android 9 and above where socket was connected but emit was not getting on server.
After couple of hours struggle finally found the problem. The problem was socket io was using http protocol and it's clearText HTTP request and it is banned in recent android version considering security reasons.
In earlier android version by default clearText is set as true but in later version default value is false. That means android components like HTTP and FTP and classes DownloadManager, and MediaPlayer can not use HTTP.
To over come this issue and allow socket to connect with HTTP protocol you have to allow clearText traffic on app level and for that you have to add following attribute in Application tag of your AndroidManifest.xml
Once you add this to your manifest file, your socket emit will start working and you can still use HTTP with your socket.
Please note that since it was testing app we used HTTP based socket. However in production you should always use HTTPs protocol.
Hope this helps you.