Hello, Archon!
I can assure you that troops cannot leave your Acropolis in case they were successfully secured. All actions in the game are made only after the appropriate commands which are sent by the player. This is why if your troops reach the Acropolis, then they can leave it only if you recall them manually.
However, the troops can be sent into the Acropolis and not reach it due to an unstable internet connection. In this case, the command is not delivered to the game server and your troops remain in your City. Please, always double check whether your troops have arrived in the Acropolis before logging off the game.
Also, we handle all requests individually. If your friends are experiencing any issues in the game, they will need to contact us personally.
Since the issue has not been classified as a bug, the topic will be closed.