Apple has also posted a support document detailing how to resolve the issue for those already affected.
Your password might be displayed instead of your password hint if you used the Add APFS Volume command in Disk Utility to create an encrypted APFS volume, and you supplied a password hint. Changing the password on an affected volume clears the hint but doesn’t affect the underlying encryption keys that protect the data.
Apple recommends that you take these steps to guard the security of your data:
1. Install the macOS High Sierra 10.13 Supplemental Update from the App Store updates page.
2. Create an encypted backup of the data in your affected encrypted APFS volume.
3. Open Disk Utility and select the affected encrypted APFS volume in the sidebar.
4. Click Unmount to unmount the volume.
5. Click Erase.
6. When asked, type a name for the volume in the Name field.
7. Change Format to APFS.
8. Then change Format again to APFS (Encrypted).
9. Enter a new password in the dialog. Enter it again to verify the password, and if you’d like to, provide a hint for the encrypted APFS volume. Click Choose.
10. Click Erase. You can see the progress of the Erase process.
11. Click Done when the process is complete.
12. Restore the data that you backed up in Step 1 to the new encrypted APFS volume that you just created.
You can download the macOS High Sierra 10.13 Supplemental Update from the Mac App Store.
[via Felix Schwarz]