Earlier quoted context omitted.
Keep in mind that this only happens if you turned it on using Disk Utility. If you did the normal upgrade (the vast majority of users) you didn't go through disk utility, and everything's fine (presumably). Still a terrible error, but affects far fewer people than your comment implied.
What if you used command line diskutil?
> When creating an encrypted APFS volume in Terminal with the diskutil command line utility, the actual hint is shown, rather than the password.
https://www.macrumors.com/2017/10/05/macos-high-sierra-disk-...