treebee If it self-starts when out of the case, then the issue isn't related to the case pressing the power button, and thus is likely hardware or software.
Unfortunately, most of the things you might try next will be disruptive, so you would want good backups. You could try sideloading an OTA image, or factory resetting, or flashing back to Google's OS.
Or maybe if you talk to GrapheneOS developers on Matrix they might have you upload logs which might be diagnostic.