This are probably 2 different issues, but since I'm not really sure I will post
them anyway...

I'm using a setup like the one explained here:
https://en.opensuse.org/SDB:Encrypted_root_file_system

I have only one disk, /boot/efi is in a separate partition and the rest is in a
LVM physical volume.

The physical volume is composed of 2 logical volumes: Btrfs for / and a swap
volume.

The first issue appears in the GRUB passphrase prompt, I would say that half of
the time, it apparently hangs after entering the passphrase. The message I see
is:
...
Slot 0 opened
Please press 't' top show the boot menu on this console

The second issue appears after successfully decrypting the master key, once in
the GRUB graphical menu, sometimes it hangs and does not displays plymouth.

In both cases, Ctrl+Alt+Del works correctly to reboot the machine.

The only workaround I know is to attempt booting several times but it's really
annoying...