Grub Themes
The problem may have been caused by a grub customizer bug. An alternative to grub customizer (a third party program) is to use grub-themes which comes standard on every Ubuntu installation:

Make Grub fonts
Even if you are just using regular grub, if you want a different font you must use grub-mkfont
program first. The same holds true for Grub Customizer used by OP and Grub Themes used in first section of this answer.
Here are the fonts I have made for grub to use:
$ ll /usr/share/grub/themes/Tuxkiller2/*.pf2
-rwxrwx--- 1 root root 176923 Jun 9 16:41 /usr/share/grub/themes/Tuxkiller2/dejavu_14.pf2*
-rwxrwx--- 1 root root 20041 Jun 9 16:41 /usr/share/grub/themes/Tuxkiller2/droidlogo_bold_17.pf2*
-rwxrwx--- 1 root root 23339 Jun 9 16:41 /usr/share/grub/themes/Tuxkiller2/droidlogo_bold_20.pf2*
-rwxrwx--- 1 root root 31498 Jun 9 16:41 /usr/share/grub/themes/Tuxkiller2/droidlogo_bold_26.pf2*
-rwxrwx--- 1 root root 15244 Jun 9 16:41 /usr/share/grub/themes/Tuxkiller2/droidlogo_regular_12.pf2*
-rwxrwx--- 1 root root 19148 Jun 9 16:41 /usr/share/grub/themes/Tuxkiller2/droidlogo_regular_17.pf2*
-rwxrwx--- 1 root root 113623 Jul 11 17:05 /usr/share/grub/themes/Tuxkiller2/ubuntu_bold_italic_36.pf2*
-rwxrwx--- 1 root root 45391 Jul 11 17:08 /usr/share/grub/themes/Tuxkiller2/ubuntu_light_20.pf2*
-rwxrwx--- 1 root root 41020 Jul 11 17:16 /usr/share/grub/themes/Tuxkiller2/ubuntu_regular_18.pf2*
-rwxrwx--- 1 root root 46423 Jul 11 17:16 /usr/share/grub/themes/Tuxkiller2/ubuntu_regular_20.pf2*
-rwxrwx--- 1 root root 68602 Jul 11 16:59 /usr/share/grub/themes/Tuxkiller2/ubuntu_regular_28.pf2*
To create a font use the grub-mkfont
program like this guide outlines: Increase Font in GRUB for High DPI Screens:
sudo grub-mkfont --output=/boot/grub/fonts/DejaVuSansMono24.pf2 \
--size=24 /usr/share/fonts/truetype/dejavu/DejaVuSansMono.ttf
Then in your /etc/default/grub
configuration file tell it to use the new .pf2
font file:
# More readable font on high dpi screen, generated with
# sudo grub-mkfont --output=/boot/grub/fonts/DejaVuSansMono24.pf2 \
# --size=24 /usr/share/fonts/truetype/dejavu/DejaVuSansMono.ttf
GRUB_FONT=/boot/grub/fonts/DejaVuSansMono24.pf2
Finally run:
sudo update-grub
Reply to comment: Grub themes are built into grub so use:
$ apt policy grub2-common
grub2-common:
Installed: 2.02~beta2-36ubuntu3.18
Candidate: 2.02~beta2-36ubuntu3.18
Version table:
*** 2.02~beta2-36ubuntu3.18 500
500 http://ca.archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
100 /var/lib/dpkg/status
2.02~beta2-36ubuntu3 500
500 http://ca.archive.ubuntu.com/ubuntu xenial/main amd64 Packages