We recently discovered that the automatic WireGuard key rotation is broken in the latest app release, version 2020.5. This affects all desktop operating systems and Android, but not iOS.
This does not compromise the security of your tunnels in any way. It just means you will not get a new tunnel IP every seven days. The reason the automatic key rotation exist is to make it somewhat harder to track and fingerprint a single user, by figuring out their tunnel IP, which is bound to the WireGuard key.
The WireGuard key, and tunnel IP, can still be rotated manually by going into Settings → Advanced → WireGuard key and clicking on “Regenerate key”.