stumpy123 The account system is a design feature of Android, so everything is operating as intended. As Oggyo wrote, the account system works within each user profile, not across profiles, so you can use profiles to isolate groups of apps.
Within a profile apps communicate (a lot) via IPC to implement normal system activities such as one app using another to render special content, transform text to speech, look up spelling, etc. These are controlled, but not banned, by the Android model and thus GrapheneOS. Over time GrapheneOS adds monitoring and control, but for complete isolation of an app from other apps you might want to run it on a dedicated device.