...have you looked in ~/Library/Containers/Keychains to see if another login keychain was created when you changed the password or were the contents deleted?
Not specifically, but I clicked on every item in Keychain Access's sidebar (where a newly created keychain would have appeared), and my missing login items weren't in any of them.
Is there a difference in the size of the populated and depopulated keychains?
Never thought to look.
Does the creation date on the login keychain change when the file is depopulated?
Never thought to look.
Have you tried exporting the login keychain contents and then importing them back into the depopulated keychain as a workaround?
Nope.
Obviously none of this solves your issue, but may help explain what is going on if not why.
I really appreciate your interest and input, but I'm throwing in the towel on this one (until 10.13.3) unless Apple asks me for more input than I've already given them.
If I wanted to work around the bug I could simply equalize my passwords before I migrate my data, leave them equalized afterwards, and be done with it, but I don't like the idea of working around this bug...any critical OS bug for that matter, because the probably unrealistic, totally paranoid specter of something going wrong with the workaround would always be lurking in the back of my head.
Nah, I'll wait for Apple to fix it. (I'm not crazy hot to upgrade to High Sierra anyhow, particularly since I intend to run it HFS+ at first and won't even reap its major benefit, and I don't feel that Meltdown presents enough of a threat to me to allow it to influence my decision.)
