Make an backups of your phone, in iCloud and if you have it on your computer.
Erase your phone to factory settings.
Set it up "manually" with no restore of backup and no extra apps.
Then see if it still does it.
If it does then have it looked at at a genius bar. https://getsupport.apple.com/locations
If it does not then some third party app is causing it.
Now either Turn off iCloud Backup in iPhone, Settings, [tap your name], iCloud, Backup until it gets fixed, or erase the phone again and restore the backup during setup.
Resources:
Back Up: How to back up your iPhone, iPad, and iPod touch - Apple Support
Erase & Restore: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
Then add back credit cards, sign back into accounts etc.