My solution is to remove custom entries from info.plist if there are any, and try again.
My app used an unsecure websocket connection, and so I added the "NSAppTransportSecurity"/"NSAllowsArbitraryLoads"entries to my info.plist. The app did not show in Testflight and I could not figure out what was wrong.
Then I moved these keys to the "Custom iOS Target Properties" section of the "Info" tab of my target, and the problem resolved.