Graphnoobie Pixel 5a is end-of-life and haven't received updates in ages. GrapheneOS added the functionality for user-facing system crash reporting so there's no reason you ever would have seen it anywhere else. It seems it happened while you weren't using the phone and you may not have even noticed that it rebooted itself. Pixel 5a is unsafe to use so you should really replace it for that reason alone. Any non-clean shutdown/reboot is considered a kernel crash by the infrastructure we're using to report them so it could be a kernel bug but it could also simply be the device's power cutting out, a hardware level lockup, all kinds of issues triggering a kernel panic without it really being a kernel bug like a CPU glitch, memory glitch, firmware bug, etc.