Emmy did you add any apps or SIM at all to your fresh clean setup?
As I wrote before:
harp Meanwhile I did a fresh install of GOS (flashed the actual image) and installed nothing else but RethinkDNS.
That was the first reinstall/reflashing of GOS after I recognized this behavior. (Did this not because of that but for other, not related reasons.) I can't really remember anymore if I added a SIM before the first "test" after this reflashing, but RDNS was the only app installed beside the GOS-installation. A few month after I reinstalled GOS again (also for other, not related reasons), but didn't care about any further "testing" of that behavior. But of course, it still happens.
The most accurate test was made by @Victus. As you can read, without any SIM or any other app installed but RDNS. It was tested with active RDNS and with RDNS completely disabled too, with traffic recorded by a Pi-hole outside the device. Both with the same result. Of course, both tests were made with RDNS already installed, but it would be curious if an installed but completely disabled app would cause this.
Emmy Only two of them flag them under Download or Download Manager, the other two as MTP Host or so.
It doesn't really mean, that the one or the other apps causes that because as written before, all these 5 apps are related to each other or maybe you can say "bundled": MTP-Host, com.android.providers.media, Sounds, Download-Manager, Downloads, as stated here. You can check this by tapping one of these connections, than tapping the related app you'll see the RDNS rule details panel of this app with it's name "+ 4 other apps" on top and a "i" in a circle symbol. Tapping that symbol will show you all these 5 apps. Also, if you define a rule (like blocking completely or a specific domain) or change a preference for one of these apps in Rethink, that's always applied to all of these 5 apps in the same way.
A few things I checked meanwhile:
Had the Idea to look at the background data usage of these apps in their App-Info panels in GOS-Settings. All of them show the exact same number of usage. When tapping on background usage to see the details, always the details panel of com.android.providers.media opens, no matter if you tap it from MTP-Host, Downloads or one of the other three. That may be a hint, that it's this one of these 5 apps which attempts the connections to gstatic.com, but not inevitable.
Also, I connected an USB flash drive and left it connected for more than 30 minutes to see, if that may happen for other USB connections beside charging too. But it did not, so this behavior really seems only be triggered while charging.
But seemingly no one still knows or figured out what's really causing that mysterious behavior or why. If it's a general AOSP issue or related to firmware or whatever, if it only happens to a few devices or generally to all, but most people may not have recognized it or if it depends on some system settings or....