When DNS Content Filter enabled company root domain won't forward to website

At two separate companies I manage, I changed their Zyxel FLEX router (current firmware) from using Web Content Filter to DNS Content Filter and things work well except the company root domains no longer forward to the company website.

Say our company is example.com , normally when you enter that in a browser it forwards to www.example.com. However when DNS Content Filter is enabled, it says

ERR_NAME_NOT_RESOLVED

If I do an nslookup it doesn't find any ip address. But if I turn off DNS Content Filter it works fine.

All other websites work fine. I do not have the domain on the blacklist and I have tried with and without adding to the safe list and that doesn't help.

EDIT: I had configured our DNS server to forward external requests to the Zyxel router because otherwise content filtering wasn't working properly. I finally realized that I had a security policy rule that was allowing our external DNS servers and so that was allowing everything to bypass filtering. Once I removed that and then changed our forwarders back to our external DNS servers instead of the Zyxel everything worked correctly.