Did you set up the restored iPad from a backup? If so, try setting it up as a new iPad instead and see if the problem persists.
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... try setting it up as a new iPad instead and see if the problem persists.
It could be data stored in one or more of the apps. Go to Settings>General>iPad Storage and then scroll down the list of installed apps to see if there are any that are shown as using way too much data.There is some data with some of the apps which would be extremely inconvenient to lose. That's why I restored the backup. Question still is, "What is this 13.64 GB of storage being required?" Sync never asked for that previously.
There is nothing over 454 MB and most a couple of tens of MB. Post #15 shows storage usage. You can see that the biggest block of used storage is 'other'. What's that?... if there are any that are shown as using way too much data.