Thank goodness that Howard Oakley and friends are staying on top of APFS bugs and security issues:
If you have erased an existing unencrypted APFS volume to change it into an encrypted APFS volume in the last 20 days or so, then you can be certain that the passphrase to that encrypted volume is stored in your unified log, and accessible to anyone who can access your Mac as an admin user (or when an admin user is logged on).
Just like the last security problem, the actual APFS format is not the problem, a Disk Utility bug is. Hopefully Apple will fix this ASAP.