It looks very weird, but if I tap the Prey icon I get the login page, and after entering the (correct) password, Pocketbook reader starts instead of Prey.
If I uninstall the Pocketbook reader, it works fine, but if I reinstall (and reboot) the problem is back.
Prey reports correctly though, and reacts to requests like update location.
Xiaomi Mi A1with Android 9.