Screen Time - Share Across Devices changes code
I can confirm this behavior. All of my devices (Mac OS and iPad OS) are getting an older screen time passcode, other than the one that I have configured on my iPhone for iOS 13.

I spent a good chunk of my day with an Apple support engineer on Friday. In a screen sharing session I showed him the behavior on my mac and iPad. I set my screen time passcode on my iPhone to "1234", and enabled "share across devices". Then, on my mac, I enabled "share across devices" and it got a different passcode than 1234. It is a bug and the only work-around for now is to reset the screen time passcode on each device and NOT enable "share across devices".

I sent them logs and have a follow up with them on Wednesday.

It is possible to reset the screen time code for Mac OS by signing out of iCloud in System Preferences, manually enabling screen time while NOT signed in to iCloud (this is important to prevent "share across devices" from getting auto-enabled), and then signing back in to iCloud. At this point, DO NOT CHECK "share across devices" in screen time settings as it will restore the old, unknown passcode.

For iOS 13, you can reset the screen time pass code by connecting your device to a Mac, making a local backup, and then clicking "Restore iPhone..." using said backup. Backups in iOS 13 do not contain the screen time passcode, and performing a "Restore iPhone..." operation completely wipes the device of all user settings and OS settings.

For iPad OS, I believe a similar approach will work but I have not tried it.