I think it's about time to lean on Apple; this issue is 7 months and 5 versions of macOS old at this point, and it's long past time for them to have at least examined it.
It may well be that Apple does not consider it an issue and the system is performing as intended.
I think that's an awfully long stretch in Apple's direction.
I'd expect an
in-your-face change such as this to be documented (and there's still a "change password" option in the Keychain Access menu bar).
I simply can't imagine Apple changing things so keychains that previously migrated without a hitch now turn up EMPTY at the end of the process with no explanation. (I mean, "SURPRISE! Your keychain is unpopulated, and it's up to you to figure out why." is just not Apple's style.)
