a3n3a I'm not saying it couldn't be a HW issue, I just find it highly unlikely.
Given a small number of incidents without repetition, it's genuinely hard to say!
Compared to Google's stock Pixel OS, GrapheneOS devices get more updates, and the default GrapheneOS settings are more "pro-reboot". If most people don't reboot their phones between updates, then a large fraction of all reboots happen at the same time as an update. If one person observes a problem after one update, it's really hard to say whether the problem is because of the new software, or because of a glitch that could happen on any reboot, or whether the reboot has shed light on a developing hardware problem.
From a "fleet" perspective, though, if there are 100,000(?) GrapheneOS users, it's pretty likely that each update will expose some hardware issue on some small number of phones.
a3n3a I've never had this issue, right up until the reboot after the latest update.
That by itself does not rule out a hardware problem, because every hardware problem doesn't happen until it starts happening, and hardware problems which are latent do often show up when a device is powered off and on, or otherwise go through a major state transition such as rebooting.
a3n3a Nothing physical happened (dropping the phone, damaging the camera in some way)
Sometimes hardware fails without a physical shock. A single bad transistor (out of a billion) can take a hardware subsystem offline.
Google might have enough data from warranty claims to predict how many devices might fail per update. People do study these phenomena at scale (e.g., Backblaze). But in this forum probably all we can do is compare hunches.