m0ng00se Although a HW defect is always possible, there was no harming incident or whatsoever.
If the flash storage had been about to fail, downloading the A15 release all in one go might have been enough concentrated load to push it over the edge. I think I remember seeing that for this release even the delta update is large (not surprising since it's a new OS). Anyway, I believe the delta update saves only download size: to do the upgrade it is necessary to combine the delta with the running system image to write an entire new system image.
And then after rebooting GrapheneOS will pre-compile all of the installed apps, which is another burst of load on the storage.
Overall, it is possible that the upgrade itself pushed the device over the edge. Hopefully not, in which case the next release might get you back in business. But it's a possibility.