This can be related to several things from software to hardware. It is also very difficult to troubleshoot the issue. You have already restored your iPhone from backup and are experiencing the same results. There may even be something in the backup file as well. Perhaps an app that is not performing well and affecting your iPhone's performance. You may have a hardware issue. The thing is that since you cannot duplicate the incident at will then it gets tougher to determine the cause.
Your iPhone 11 is less than one year old and is therefore covered by the Apple 1YR Limited Warranty (US) so have it examined at the Apple Store or at an authorized service center and take advantage of it. If the stores are closed then please contact ➔ Apple Support
Axel F.