Baptiste_Milton ok, one simple check you can do is the following;
At first bootscreen, the one with the pause option top right', press pause.
On this screen there should be a long line, and for GrapheneOS Pixel 6, it should be exactly this line: f0a890375d1405e62ebfd87e8d3f475f948ef031bbf9ddd516d5f600a23677e8
Reboot, pause, check if this holds true for you, then report back to us.