If you lose your Google Timeline data for no appearent reason, and you have more than one device, then try the following:
3. Personal Content
4. Web & App Activity
5. See All Activity Controls
6. Location History must be 'On'
7. One line lower you'll find 'Devices on this account', click this
8. Leave tracking on for only one device (unfortunately it doesn't show the nicknames)
It seems GoogleMaps gets confused when dealing with multiple devices. You would expect it to be able to track more than one device? And why hide that option, several menus down?!?
Update. This acutally works, yeah!