How do I fix Screen Time turning off automatically on iPhone?

Did you enter your Apple Account/Password as a reset option to the Screen Time Passcode? You are given that option after setting a Screen Time Passcode and can skip it to prevent the Apple Account/Password reset. If you did, that is all they would need to reset the Screen Time password themselves and turn it off.

Have you allowed them to make Account Changes, so they could simply sign out of their Apple Account. Check that at Screen Time > Content & Privacy Restrictions > Account Changes. It should be set to Don't Allow.

Does your child have access to a computer to completely reset their phone. It will result in the phone being completely wiped of its data. It is the only way that the Screen Time Passcode can be removed when it has been forgotten and no reset option has been given. If there was not a failsafe method such as this, then a device could forever be locked with those restrictions, even if you forgot the Passcode.

Also updating all devices to iOS 27 gives you far more options with Screen Time, since it has been totally revamped in that version.

And finally, technology is never a replacement for parenting and knowing what your child is doing online and with their phone. If a child is willfully going to disrespect your decisions and go out of their way to disobey your clear instructions, they don't deserve an iPhone. Get them a "dumb" phone that is only capable of phone calls, yes they still exist. Most kids would be so embarrassed to be carrying one of those around that they will be more willing to earn your respect when they realize the alternatives.