I'm also having the same issue. I used Pandora in my car 3 times for about 15 minutes each and it used 4GB!
I have the latest version. I do not download offline stations. I have the "high quality audio" setting turned off.
When using CarPlay, Apple doesn't make you open the Pandora app on the actual iPhone. It appears on the screen of my vehicle and can be launched there. The only thing I can think of is that CarPlay keeps Pandora open in the background (even though the app is not open on my phone when I disconnect it from the car).
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