The Google Maps Timeline has long been a useful though slightly uncomfortable feature that maintains a complete record of everywhere your phone goes (and probably you with it). Google recently changed the way it stored timeline data to improve privacy,

At first glance, reports of people’s timelines being lost seems like they might have fallen afoul of Google Map’s big privacy change: your Google Maps timeline is moving from the cloud to your device, and unless you agree to various new settings by a deadline date ( which is different for different users, weirdly) you’ll lose your travel history.