lots of timezones are missing from the timezone dropdown list

I want to set system timezone to Canada/Eastern. It's not in the timezone dropdown list for selection. A lot of timezone are missing from the dropdown list.

I found timezone America/Toronto from the following command:
timedatectl -list-timezones

So I set my system timezone to America/Toronto via timedatectl command. All was good, time was displayed correctly after set. But after I reboot the system, the system timezone was reset to a randomly picked Pacific/Kiritimati.

I manually changed the /etc/localtime via ln command to point my system timezone to Canada/Eastern, I got the correct time displayed right after the change. But after a while, /etc/localtime was changed to point back to Pacific/Kiritimati for unknown reason. No system reboot before the change back happened! I've only installed one or two new packages, like screenfetch.