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Lumia 1520 running Windows Phone 8.1

For the last three weeks, my phone has suddenly been burning through my data plan. In Data Sense, the culprit is "Restore Data".

At the end of my last month, Restore Data had used over 400mb of cellular data and drove me to my 1gig limit for the first time ever. It didn't come to my attention until my 1520 prompted me with a warning that I was on the verge of exceeding my data limit.

After two restarts, this new phenomena is continuing. I have restricted all background data use, which stops Restore Data from continuing to drain data over cellular, but if I'm on WiFi, it continues this behavior using WiFi data.

In 12 days, Restore Data has used 61mb of cellular (before I restricted background data), and 840mb of WiFi data (which would have been almost all of my cellular data if I hadn't restricted celluar). In the last 24 hours, it has used 67mb over WiFi.

The only app update prior to this problem was Weather on November 24th, and weather is only showing <1mb of data use in Data Sense.

Could this be a Microsoft "back end" change at some point in December, affecting Windows Phone 8.1?

Does anyone know exactly what "Restore Data" is doing?

Is anyone experiencing this problem? Restricting all background data is a poor stop-gap measure because I no longer receive email or social media notifications unless I'm on WiFi.