I've seen this before, and it was due to a corrupted iTunes backup.
Here is my suggestion.
1) [https://www.imore.com/how-reclaim-lost-gigabytes-hard-drive-space-itunes|delete old iTunes Backups]
2) double tap the iPhone 7 home button, and make sure no apps are currently running, then reboot the phone.
3) create a new iphone 7 backup in iTunes
4) double tap the iphone 8P and make sure no apps are running and reboot the phone
5) restore the backup on to the iphone 8P from step 3
6) On the iphone 8P, after backup succeeds, go to settings --> General -->Reset --> Reset all Settings
You should be good to go from here.