Same problem here, i already published in "Feedback" a detailed information about.
In my case i already configured "screen time" in 17.5.1, then when i upgrade to 18 public beta its stop working. Apps freezes by natural limit time, BUT i cant unfreeze,the option "unlock for 15 minutes" and "unlock all day" not works.
and "screen time" in "setting" freezes for long time when i enter in, and theres another new option "sites and app activities" supposed to reenable , but it crashs