robzlatarski If it's hardware then it's a massive coincidence that it just started happening after the phone rebooted from the update.
I have read that there are more than 250,000 GrapheneOS users. Any time a fleet of 250,000 devices reboot some low-probability events will be observed, especially if those devices typically don't reboot often.
If we had 10 or 20 complaints about power buttons immediately after the update, I suspect that would trigger a rapid investigation by the developers. But it seems as if the anecdotes we have so far are two recent power-button reports that are not related to an update and one that may be, but also may be a coincidence (which would be my hunch).