What is Marshmallow encryption protecting?
Because I still use Shamu as personal device, I had experience with CM ROMs.

Nexus devices with L or M had forced encryption at first bootstrap - that is, forceencrypt flag is present in fstab.qcom, so your device is automatically encrypted.

In those cases, master key is encrypted with default password, which is default_password

Most probably, when you had set lock screen PIN, you haven't selected Require PIN to start device, hence, there is no preboot authentication.

In regard to FDE, in AOSP ROMs only /data is encrypted; but there are solutions for LUKS or PBE that are not user friendly. Several years ago, WhisperCore was launched and abandoned.