boot - LUKS passphrase screen does not show up

I installed Ubuntu with a fully LUKS encrypted LVM.

It works fine with one issue:
After selecting Ubuntu in Grub while booting, the "enter passphrase to unlock disk" screen does not show up. I can (blindly) type my password into the black screen and it works. After a few seconds the normal boot stuff happens and everything is fine.

But I wonder why I get nothing on my screen.

The first 2-3 times after installation it worked as expected but at some point the over the last few days the screen stayed black. (I can't really say what I've done that caused it..)

This is my /etc/default/grub file:

GRUB_DEFAULT=0
#GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true

GRUB_TIMEOUT=10
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="kopt=root=/dev/mapper/vgubuntu-root quiet splash"
GRUB_CMDLINE_LINUX=""

# Uncomment to enable BadRAM filtering, modify to suit your needs
# This works with Linux (no patch required) and with any kernel that obtains
# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"


# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console

# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo'
#GRUB_GFXMODE=640x480

# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
#GRUB_DISABLE_LINUX_UUID=true


# Uncomment to disable generation of recovery mode menu entries
#GRUB_DISABLE_RECOVERY="true"

# Uncomment to get a beep at grub start
#GRUB_INIT_TUNE="480 440 1"