Hello there and welcome to ZenTalk!
The current firmware version is WW-16.1220.1909.194. You can download it from here.
https://www.asus.com/us/Phone/ZenFone-6-ZS630KL/HelpDesk_Download/The restart issue was aggravated by that FW update, but not caused by it. Updating to the most recent firmware is always recommended for security and stability. Update notes can be found on the same website I linked you.
Please update to the most recent FW and see if your phone starts functioning normally. If your device continues to restart abnormally we recommend contacting your local service center.