Are you on the latest firmware version, 220.127.116.11?
You can check that following:
Settings>System>About Phone>Software information
2. If you are on the latest then I'll suggest you to check on SMMI test:
Open Calculator> type ".12345+=">SMMI TEST>Single Test>Front Camera Capture Test
If any of the SMMI tests fails, indicating high possibility of faulty hardware, kindly bring your device to the nearest ASUS repair center for device checking: https://www.asus.com/support/Service-Center-All-Countries/