I have had this incredibly frustrating problem twice now -- I've actually had two problems: 1) not all apps are blocked at downtime, and 2) apps aren't blocked after the time limit is reached. Finally, after HOURS of time trying to fix the problem, I finally figured out something that worked! In the hopes that this might help others, I wanted to post my solution.
Step 1: from the parent's phone, turn off screen time for the child's account. Settings->Screen Time->Child's name->Turn Off Screen Time (scroll to the very bottom to find this)
Step 2: from the child's phone, go into settings and sign out of the phone. Settings->Child's name (at the very top)->Sign Out (at the very bottom). This didn't change any of the contents of the phone (at least it didn't for me), it just removes your child's apple ID from the phone.
Step 3: (this one might not be needed, but I did this part too, so just in case) power down the child's phone and keep it off for one or two minutes.
Step 4: power the child's phone back up, and sign back in using the child's apple ID. Settings->Sign in to your iPhone (at the top).
This completely cleared all the Screen Time settings from the phone and reset it back to what it looked like when everything was new. From there, I just went through the normal steps for setting up Screen Time for the first time. When it was set back up, the problem with screen time not blocking apps was gone ... at least for now.
My child only has one Apple device (the phone), so I don't know if it would be more complicated with more devices. Maybe you have to do it for all of them simultaneously?
At any rate, good luck -- I hope this helps!