To factory reset your iPhone 8, follow these steps:
- iTunes or iCloud may be used to back up your personal data.
- Open the Settings application.
- Select General.
- Restart by pressing the Reset button.
- Erase All Content and Settings is the last option.
- Enter your passcode if prompted.
- Erase your iPhone by pressing the Erase button.
Now that you’ve factory reset your iPhone, set it up as a new device (and not from your backup). Then, use Facetime again and replicate the issue. Make sure that you don’t install anything during this time.