Venturing a guess, might be same bug that allows downloading more than 150MB apps over cellular (thread on this sub board on it). Turn off automatic time setting, roll back the date/time. Reset back when done.
Turned on screen time and changed a couple of settings to "Don't Allow" and was still able to manually set date/time.
Like I said, a guess, as did not want to dive deep into Screen Time and Restriction settings.