diaboliquepsychotea Hi there!
So, a couple of things here. You are probably talking about the blog by Mullvad that's talking about connectivity checks not going through the VPN (source).
While this blog isn't wrong, it's kind of misleading. This is not a leak because it's intended behavior. Connectivity checks have been designed to bypass the VPN.
That said, GrapheneOS does and had provided a way to disable connectivity checks for a very long time now.
That option is documented along with other default connections and what they're for in the project's FAQ, found here:
You can change the connectivity check URLs via the Settings ➔ Network & Internet ➔ Internet connectivity check setting. At the moment, it can be toggled between the GrapheneOS servers (default), the standard Google servers used by billions of other Android devices or disabled.
I hope that answers your question!