Turned out, all apps from this vendor talked to each other, in the background. If one app has filesystem access but no network access, and another has network but no filesystem access, the former can upload private filesystem data by sending it through the latter.
Further suspicion: Apps remain killed, for long periods of time, if I don't start any of them.
Quick test: Kill all apps. Start them one by one. Check if other apps are now running.
Confirmation: Pull APKs from device; RE their code for IPC.
You’ve given me something to think about. Luckily, I only have to amend my mental model a bit, to assume giving a permission to any vendor’s app is to give that permission to every app from that vendor. In most cases where that would be a problem, I already run such apps under a separate user profile, which fully prevents IPC.