This wasn't a issue in the previous versions of TeamViewer however is missing this option in the version 13.1...
How can I autostart Teamviewer minimized to tray? (Even with the contacts).
@AiOGajo, that is the default behavior. If TeamViewer is started by the system (not manually), then it should be in the background.
The only reason it will start in the foreground is, if no tray icon can be created.See System Tray, StatusNotifier and TeamViewer for Linux