The only reason this was detected was very overt behavior - opening AD popups. So I guesstimate for each one of these we have 10 that go undetected. This means the whole ecosystem is broken, as there is no reason this will happen only for updates and not for new apps as well. Apple's ecosystem is somewhat better, but I can't imagine they go through every line of code in each package, so most of their review is probab…
I had two apps that radically changed their business model (owner?) through updates with no recourse.
I had an app called gas cubby, which let me locally - on the phone - keep track of all my vehicles. I could enter detailed information about each car such as year, make, model, vin, insurance policy, gas purchases, oil changes and the like. It would tell you gas mileage and remind you of upcoming maintenance. One day, I updated the app and all my local data was uploaded to the cloud.
Another app I updated was camscanner from tencent that basically did the same thing. Think of all the PDFs you scan going to their cloud.