Running iOS 9.3 (13E5181f) on iPhone 6s Plus and iPad Air 2, the cellular data usage is through the roof. Between the two devices they have consumed 38 GB of data on LTE AT&T in two weeks (nominally 2-5 GB in the same period combined). The culprit appears to be Cellular Data -> System Services -> Documents and Sync. Tried erase & restore, reset network settings, reset all settings, disable all Internet accounts including iCloud - no changes. System logs look normal to me, though not sure what I would be looking for. No cellular usage when connected to WiFi as expected. Any thoughts?
iOS 9.3 Documents and Sync data usage
See this frequently in the system log :
Jan 24 13:28:22 xxx syncdefaultsd[1266] <Warning>: UM: (501) ADD USER SYNC TASK
Jan 24 13:28:22 xxx syncdefaultsd[1266] <Warning>: UM: (501) ADD USER SYNC TASK
Jan 24 13:28:22 xxx syncdefaultsd[1266] <Warning>: UM: (501) REMOVE USER SYNC TASK
Jan 24 13:28:22 xxx syncdefaultsd[1266] <Warning>: UM: (501) ADD USER SYNC TASK
Jan 24 13:28:22 xxx syncdefaultsd[1266] <Warning>: UM: (501) REMOVE USER SYNC TASK
Check the release notes and then file bugs to see what comes back.
My money, tho, is on the fact that it is an early beta where so many things are held together with hope and superstition.