geolocation – Recovering missing Google Timeline data

Overall, Google Timeline is a great record of my movements and I’ve found it invaluable in figuring out where I was at various times. But once in a while, the timeline will say ‘no data’ for a given day (and show the world map instead of my location to confirm it has no location data for that day). In other situations, it will show my ‘home’ location but nothing more, as if I never went anywhere that day (but I ALWAYS carry my phone, and it is ALWAYS on). Does anyone know why this happens?

I have found a way to ‘recover’ the data, which demonstrates that the data really is there; something is causing it to be hidden. To recover the data – first, I went to settings / ‘show raw data’; no data appears (no red dots, even if I zoom way in to the location I know I was at). Next, I choose the option to ‘add location’, and enter my home address, and a timeframe I know I was at home that day. Now, magically, a whole bunch of ‘movement lines’ appear! And now, if I choose settings / show raw data’, a whole bunch of red dots appear. From this point, I can use the red dots to remind me of where I was, and use the ‘add location’ feature to re-create the movements of the day. But why did the day initially show as having no data, and why did manually adding my home address suddenly reveal all the raw data for the day?