You only need to turn on data in order to send/ receive photos but no data usage will be charged. I keep data turned off otherwise and use wifi to download apps.
It wasn't a "bug" when data usage went up, nor was it "fixed" when the use disappeared. That's how it works. Again you must turn on data to send or receive photos or videos but not necessary just to text someone. Use wifi for most apps and browsing the web when in a hot spot and use data for when away from home or hot spots.