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1. Check your iCloud settings (also your messages and if you don’t need - delete all the messages there stored in, and after that disable it. Turn off and on the phone.
I've already tried disabling messages in iCloud, the drain was basically the same (tested over few days).

2. Check iCloud Drive settings (maybe there is an app syncing data the whole time) - I had a case where an older App that stored data was in endless sync with it.
I'll try disabling all apps in the iCloud settings.

3. Check if you use any DNS ADBlocking features - in some cases if you use special DNS services like NEXT DNS or Adblock with lots of blocking lists that leads to endless request from iCloud services that won’t sync anymore = battery drain like hell!
Nothing like DNS blocking is in place on the device and also not in the network I'm connected to.

4. Save your data on your iCloud or on a Mac - make a Backup.

5. Reset your phone DFU - set up as new phone without restoring any iCloud data yet.
When set up - just use your phone for one day or two and check.
That's the very last resort. It would take weeks to get everything setup again (because of some apps a hardly use but may need in case and to configure them again I need to get in touch with some ppl and also be there in person, security rules ...)
 
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