"No key available with this passphrase" after upgrade to 18.04

[A. Martos]

XXX@XXX-laptop:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04 LTS
Release: 18.04
Codename: bionic
XXX@XXX-laptop:~$ uname -a
Linux XXX-laptop 4.15.0-20-generic #21-Ubuntu SMP Tue Apr 24 06:16:15 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
XXX@XXX-laptop:~$ apt-cache policy cryptsetup
cryptsetup:
  Installé : 2:2.0.2-1ubuntu1
  Candidat : 2:2.0.2-1ubuntu1
 Table de version :
 *** 2:2.0.2-1ubuntu1 500
        500 http://fr.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
        100 /var/lib/dpkg/status
XXX@XXX-laptop:~$ sudo su
root@XXX-laptop:/home/XXX# echo -n 'My_Passphrase' | cryptsetup luksAddKey /dev/sda3
Aucune clé disponible avec cette phrase secrète.
root@XXX-laptop:/home/XXX# echo -n 'My_Qwerty_Weirdish_Passphrase' | cryptsetup luksAddKey /dev/sda3
Aucune clé disponible avec cette phrase secrète.

Since upgrade from kubuntu 17.10 to kubuntu 18.04, I had the problem described in this post :

https://askubuntu.com/questions/1031302/ubuntu-with-full-disk-encryption-bad-password-after-upgrade-to-18-04

My successful workaround was to type my passphrase as if I had set it up from an azerty physical keyboard with a qwerty software layout. No big deal (and, I think, not the same bug ?), although I wanted to luksAddKey my true passphrase so I could avoid the bother.

However, when I gave the passphrase(s) to the prompt (as shown above), cryptsetup wouldn't recognize any of them, either the true one (azerty physical and software layout) or the weird one (azerty physical layout / qwerty software layout).