momo55 The first eight account creations succeeded; then account creations failed. Proxies were alternated during the test.
You are not saying what about the account creation failed?
momo55 This suggests Google Play Services may not be fully sandboxed and could be collecting device-specific information that enables fingerprinting. Another possibility is that the proxy connection via RethinkDNS leaked and exposed the local IP address. Either way, the result is concerning.
And another possibility is that the IP addresses were simply pre-marked by Google as being suspicious or blacklisted. You are saying that the quality of the proxies are good. What is it about them that is "good"?
The GrapheneOS and GmsCompat source code is there for people to inspect. If someone can pinpoint aspects of the code which makes Play Store escape from the Android app sandbox, I'm all ears.
There can be numerous explanations to Google account creations failing, and it's not clear to me why a sandbox escape/failure would be one of the most likely conclusions to ponder here. VPN leaks are of course possible. Are you able to demonstrate one?