Please keep in mind that ecryptfs-mount-private and mount.ecryptfs_private are slightly different things.
Since ecryptfs-mount-private is a frontend to the underlying mount.ecryptfs_private, and this isn't working either (also not under 18.04), it is vital to focus on fixing that.
I found that keyctl shows different things under 16.04 and 18.04 after doing ecryptfs-add-passphrase.
Please keep in mind that ecryptfs- mount-private and mount.ecryptfs_ private are slightly different things.
Since ecryptfs- mount-private is a frontend to the underlying mount.ecryptfs_ private, and this isn't working either (also not under 18.04), it is vital to focus on fixing that.
I found that keyctl shows different things under 16.04 and 18.04 after doing ecryptfs- add-passphrase.