abdullah and they asked me to restore the phone to stock OS and put it into repair mode.
The instructions on Google store told me to do that, but I assume that's just best practice to make the phone available for technicians to test without having to reset it and then people being annoyed that they lost their data. Or something, I don't know.
My problem was that the issue was intermittent, so it would last for several hours or days or whatever, then not appear again for a while. I flashed back to stock OS the first time it happened, only for the issue to disappear for a while. Flashed back to GrapheneOS, everything was fine for several days, then when the issue appeared again I just shut down the phone and shipped it for repair; didn't want to be told by Google "hey, we're not able to reproduce the issue, here's your phone back", so to make the issue apparent for them immediately I didn't flash back to stock OS again. Google doesn't seem to care if the phone is in repair mode or not, but some local technician might see it differently.