The screen time only works randomly. I have had the same problem, regardless of what settings I use, sometimes my kids can use the computer all day.
By default we have it set to "blocked all day", and we manually use the "add time" feature to give them time if they have behaved, done homework etc.
This is really a major screwup that this is completely unreliable, it causes us untold problems, allows our kids to break the rules and order we have setup.
Not to mention the fact that the ability to "add time" has been removed on Windows 10 entirely, so we cannot upgrade any of the kids computers.
If the screentime only works randomly, you have to assume everything else is not reliable either, and they can probably use apps and access adult websites too. So you need to test this on a daily basis at the moment.
It is beyond a joke.