First and foremost, you need to update iOS. iOS 12 is 5 years old. It did receive some security updates in Jan 2023, but anything patched since with iOS 16/17 is still vulnerable in iOS 12. I would not be spending time testing anything on iOS 12 in 2023/2024.
Screen time is not a management solution. Screen Time is a tool that monitors and reports on your usage with some hooks to parental controls. I would not call Screen Time crap if it's not designed to do what you are wanting to do. I personally don't find Screen Time very useful, but I would not call it crap. Use the right tool for the job, or have a bad time.
what’s up with this?
In any event. To answer your question, I am going to wager you don't have the blueprint setup correctly. The device needs to be supervised, and you use the Wi-Fi Safest payload. On a supervised device, with this payload deployed in a Blueprint, it will restrict the WiFi networks to only MDM configured networks.
