I'm noticing this as well, and searching for a solution.

If you push a config profile with "Allow modifying restrictions (supervised only)" unchecked, it'll clear the local Screen Time restrictions (and disable Screen Time). With the combination of a Pop-up Menu extension attribute and a smart group you can setup an easy way to deploy that profile and then remove it.

I noticed this issue as well and saw some other threads about this. This is something you need to contact apple about so that can make more features around screen time (and restrictions which is inclusive to it) available for MDMs lke JAMF. If you have an apple developer account, contact them that way.

There's a FR in about this already (https://www.jamf.com/jamf-nation/feature-requests/8025/screen-time-restriction-settings-with-ios-12-it-is-really-required) but the initial changes will need to come from Apple first before MDMs can do anything.