translated from the german link:

But the police officers are doing much more than they say. They gain permanent access to WhatsApp accounts. They are setting up WhatsApp Web on a police computer. They scan the QR code for verification in the parents' WhatsApp apps. From this point on, the police can also read all future WhatsApp messages – completely without Trojans.

If you don't need your phone when you travel: burner. If you do need it: switch it off. If you visit the popo, leave your phone at home.