Hi all,
I think I have figured out a way around this. The only problem I have (for now) is that I have to keep doing it every night, so it's a bit messy. Still, it's better than nothing but I'm trying to find a way of making sure the settings stay applied.
Go into 'Screen Time' and at the bottom of the screen, select 'Turn Off App & Website Activity'.
Restart the phone.
When the phone comes back on, go back into 'Screen Time' and under 'Limit Usage', select 'App & Website Activity, then in the next window that appears, select ' Turn on App & Website Activity'.
Then, from the 'App Limits' function, Add Tik Tok and Snapchat (you can just select all Social Media apps or you can select them individually from the drop-down list).
Once you have selected the apps, select 'Next' at the top right of the screen. I think you have to set the availability for at least 1 minute. Make sure 'Block at end of Limit' is on. Then, select 'Add' at the top right of the screen.
Finally, go into 'Downtime' and set the time that you want the phone to lock. Once the phone locks at the selected time, you should see that Tik Tok and Snapchat are now blocked.
Hope it works for you!