I recently installed grapheneOS and also have sandboxed google play services installed.
When I randomly looked at the home screen there was a pending app greyed out appearing in the top left of the screen. Looked like the "android device policy" app icon or "google play services for AR" app icon. I ended up longpressing on it and sliding it up to "remove".
I noticed it after logging into the standard notes app and soon after receiving an email notification for the first time.
Now I'm wondering if that was a bug or if it was something nefarious going on. Maybe google play services tried to install an app on it's own accord? Does something auto install the first time you get a push notification?