I’ve found the solution to this question at discussions.apple.com Here is the link: Documents & Sync High Data Use Despite Turning Off iCloud Sync & Using Low Data Mode - https://discussions.apple.com/thread/250900105?answerId=251747400022#251747400022 "I found the same thread on Stack Exchange!  After exhausting my troubleshooting ideas, I got in touch with Apple Support. They connected me to a Senior Engineer who remoted into my phone to help diagnose a fix.  Ultimately what fixed it was logging out of iCloud, doing a hard reset of the phone while logged out, then logging back in to iCloud. I'd tried those things in different orders, but I had not tried a hard reset WHILE logged out of iCloud. Since logging back in on Wednesday night, my data usage has returned to normal. 3 total MB used over the last three days, CONSIDERABLY better than the 200-300 MB/day it was using. So if you're having this problem, I recommend trying this fix!"