Screen Time not accurately reporting device use and not blocking apps

I have Screen Time set up for my device (for reporting purposes) and for my kids' devices (for reporting and device use management). In particular for my kids' devices, I am seeing wildly inaccurate time reports, with a lot of time reported by Screen Time that is not allocated to any app or category, sometimes as much as 80-90% of the time that Screen Time is reporting.

For instance, if you look at the time that Screen Time says my daughter used her phone (an iPhone 6s) last Saturday (January 25th, 2020):

You can see that there is 5 hours and 46 minutes of reported device use. However, about 90% of the bar is grey. I realize that the bar is only colored with the three biggest categories, but still, these only add up to 28 minutes, so there HAS to be some app in some other category that has been used much much more.

In fact, if I look at the time by categories:

You can see that, outside the biggest three that show in color in the bar, all other categories (including the 'Other' category) only adds up to 12 minutes. Even looking by individual apps (even if every app should fall under a category, even if it's 'Other'), about 90% of the time Screen Time is saying this device was used is totally unaccounted for:

I have app limits set up, including a 3 hour limit for "All Apps and Categories", which should have essentially blocked all apps after 3 hours of use, but still she managed to use her phone for 5 hours and 46 minutes, except Screen Time has no idea what it was used for.

This is frustrating, and makes using Screen Time quite useless. I am running iOS 13.3 in both my device and my daughter's. I've also seen similar issues (although not as severe) with another one of my son's devices (for instance, out of 2 hours and 4 minutes of device use reported on Wednesday, January 29th, only 51 minutes are accounted for under some category or app). This device is an iPhone 8 also running iOS 13.3.

Ironically, the feature seems to work best with my other son's iPod touch, which cannot be upgraded to iOS 13 ad is therefore still on iOS 12, although there is still some time unaccounted for every day.

I have seen issues like these reported by Screen Time users since the feature came out with iOS 12, and frankly, it's quite frustrating to see that we're now on iOS 13.3.1 and nothing seems to have been fixed on Apple's part.