Scroll to the bottom of a list, there's a "Load more" button, here's how it works: when the list originally loads, a number of most recently changed tasks are retrieved, then every time you click "Load more", an additional bunch of most recently changed tasks are retrieved.
The problem is, "most recently changed" is not the same as "top on the list", very old tasks can be on top too, as long as they're created a long time ago and never touched since, but they're not guaranteed to be retrieved on the first go. It's more often an issue when you sort by date, because past due tasks are showed on top.
The easiest workaround is to click "load more" a few times, then don't close the app unless you need to reboot your computer.
Most user don't encounter this issue very often, because the initial bunch is often bigger than the entirety of their unfinished tasks.
You're more likely to get this issue when you recently checked off a ton of tasks, because completed tasks are retrieved together with unfinished ones, so they take up a great number in the initial load. As you gradually add more new tasks, this effect will fade out.
All this is due to how Google API is designed, not me...