Good question but some details might be helpful:
- What specific version of High Sierra were you installing: 10.13, 10.13.1, 10.13.2, 10.13.3, or 10.13.4?
- Were you instaliing from a Recovery Drive?
- Were you migrating the accounts from another bootable drive, a Time Machine backup, a disk image file, or something else?
- Was the migration performed during the initial installation and setup or did you run Migration Assistant after the installation was complete?
- Do I understand correctly that you had to create and verify new Admin passwords for all user accounts before you could migrate either of them?
- Did you migrate both accounts at the same time or separately?
I can't even begin to hazard a wild guess why Apple would have instituted this new behavior.
I think it is a safe bet that it is security related.
More: And this just occurred to me: Why would macOS allow ONE user to change the Admin passwords for TWO accounts without authenticating?
Maybe my morning Starbucks has not yet reached my brain, but this seems to contradict what you are saying happened? Is this a rhetorical question or another subject?
In the meantime I am going to brew another Starbucks and see if that clears the fog.
