Edited timeout in /etc/default/grub, and ran update-grub, but timeout remains 10sec

Question

Xubuntu 16.04, Grub 2.02~beta2-36ubuntu3

I edited the timeout to 3 seconds:

GRUB_TIMEOUT=3

Then run:

sudo update-grub

Timeout during grub boot is still 10 seconds. What am I doing wrong?

Edit: Suggested duplicate question's answer does not address the specific issue that GRUB_TIMEOUT and GRUB_HIDDEN_TIMEOUT cannot both be enabled simultaneously, and therefore to change GRUB TIMEOUT from its default setting, GRUB_HIDDEN_TIMEOUT must be commented out.