Sticky app data

Here's a minor mystery. Maybe someone can shed some light:

I force-stopped the Reddit app and nuked its cache and storage space. When I restarted it and tapped on "Log in", lo and behold, it had remembered my account.

How does that happen? Is there some hidden storage space available to apps that can never be erased? Can apps write stuff in their installation directory to survive a complete cleanup?

I also looked around in the filesystem in an adb shell, to see if it had written a file somewhere, but I couldn't find anything.

In the end, I uninstalled and reinstalled the APK and it finally forgot my old account.

I've never seen an app do that. If clearing the cache and storage space doesn't guarantee that an app is fully reset, I'm gonna start uninstalling/reinstalling them as well.

This happened in Android 13 (CalyxOS).