I've recently reinstalled Pinterest on my iPhone and noticed I was logged in right from the start without even asking me for my username/password, so it must've somehow kept some data on my phone despite the app uninstall/reinstall.
I checked the obvious stuff and it's iCloud Drive as the app doesn't even appear in the iCloud Drive settings so it can't be storing data there. It's not iCloud "look me up by email" either as the app is not listed there either (in fact nothing is listed), and it's not browser cache/cookies as those have been cleared many times after the app was first uninstalled. It's not the iTunes advertising ID either as "limit ad tracking" is enabled and the ID was reset anyway.
Where is this app persisting its data, and how do I prevent this behaviour?