This is also happening to me. I don't think it's a simple 'out of power' issue. My phone seemed unable to charge, steadily lost power, and died. It was entirely unresponsive so I left it plugged in to charge and managed to hold down the volume+power and that got me into the boot screen. It had fastboot turned on, which seemed unusual as I hadn't noticed it previously. If allowed to resume booting, I get the G logo, the GrapheneOS logo, I will very briefly see my lock screen and then the phone 'dies' again. The power bar in that brief moment shows at 100%. I've gone through this several times and it's the same every time. I'm going to try to find instructions for a recovery install, but will be without a device until then. Has there been an update pushed recently that might have caused this?
To note: I've not installed, (chosen) to update, or customized my device in anyway beyond installing the standard Graphene OS image. The device is a Pixel 9a.