BobHanson not inspiring much confidense, when that sort of thing happens.
It makes sense to think that but that's actually how it's supposed to work. GrapheneOS uses A/B seamless updates, which means when your phone is booted to one of the slots, A or B, the update is installed to the other inactive slot. When the phone restarts, it switches to the slot with the updated version. If it boots, good. If it doesn't, that means something went wrong with the update, and the phone goes back to using the slot that was working before. (More info here if you're curious.)
This actually makes the update system more fault resistant than the way phones used to update. If done the old way, all the GrapheneOS users who had to attempt updates 2+ times would have likely had bricked phones, but they don't. If anything, it should inspire more confidence since not every OEM or OS does updates this way.