I’m pretty sure I know what this will be. And the next update, coming in the next few days, should hopefully fix it!
Basically what I think is happening is the daily(ish) background task that updates the “Simple Items” used on the monthly and yearly timeline maps is backlogged and not being given enough time by iOS to catch up.
In the current Arc App build, Arc will attempt to fill in missing Simple Items when you view those maps when the app is in the foreground, if plugged in to power. But it only attempts to fill in missing items, not update existing but out of date items. The next Arc release will do both.
Keep an eye out for an update in the next few days!