The rationale behind Apple's decision to enforce user and login keychain passwords to match someone familiar. with Apple's security changes will have to explain.
No such decision has been made. The option to change your login keychain's password still exists in High Sierra (see below and your own Edit menu), and had it been eliminated, I'd expect to have found a popup advising me that my passwords had been "equalized" the first time I booted into High Sierra...NOT a mystery.
I have wondered whether your issue is actually the password difference or just perhaps an obscure glitch in your login keychain file structure itself. If you are still curious and have time for some experimentation you might try:
- To potentially eliminate the password as the issue:
- change your login keychin password to match your account password.
- Run the update/upgrade and see if the login password has been depopulated
- If it has NOT been depopulated then the issue is more likely a formatting or other glitch in the login keychain file.
I've done that...spectacularly unsuccessfully.
Well, I just did a clean install of High Sierra and migrated my data from my boot Sierra volume after changing my keychain password to match my login password, and all was good...for about 5 minutes: my keychain was populated.
I then changed my keychain password so it differed from my login password, shut down, started up, and found that my changed password had reverted and my keychain was depopulated.
