No it hasn’t. I just trialed it and it automatically unlocks everything just as it was nothing. There are a few ways to catch if someone changed the time however.
In messages, if a text was sent or received under the changed time it will show that date it arrives. For example, if I change the time for two days from now, receive a message, it will still show that two days from now date even when I change it back to the current time.
It also resets the usage when they go from one day to another and doesn’t log the time properly either.
One way to truly block it until they fix the problem is to allow a one minute “free” unblocked time. So the kiddos will have to change it every minute as it will block them. Pretty cruel and crude way of doing it, but screen time is useless until they fix it.
I noticed this bug in IOS12 Beta for my daughter's Iphone 5S. By changing the Date/Time to Manual and set to a future date, ScreenTime limits became useless.
After updating to IOS12 Final Release Version I notice that the button to turn off Automatic Date and Time is disabled. I think this has fixed the problem since the Date Time cannot be changed now and remains fixed at automatic.