GraphTester ME and a friend bought a Pixel 7a...
As far as I know, the issue with devices displaying a black screen are only 8th generation devices. You can see the official project account said as much in this post. You and your friend may have a different issue than the one that's being discussed in this thread.
GraphTester the tricks to reboot are only for stock-firmware
What tricks are those?
GraphTester but with GrapheneOS there is no chance to get it back booting!!!!!
Other people with the issue with 8th generation devices and the display issue have been able to force power off their devices by pushing and holding the lock/power button for a long time, something like 30 seconds. Have you tried this?
GraphTester so i think the actually firmware-patch of Graphene OS got a big fail and kills your phone!
GrapheneOS cannot make modifications to the firmware. If you think it's the firmware, not some other issue, then the same thing could have happened on the devices if they were running the stock OS.
GraphTester but think the new patch should be very good controlled & tested before role out!
All updates are tested thoroughly before they're released. They're tested internally, then GrapheneOS users who choose to test releases while they're still in the alpha or beta channel. Issues with new releases are almost always caught during alpha/beta testing.
So, you and your friend don't have 8th generation devices, they worked for days before the issues occurred, and (as far as I know) there are no reports of the issue you're experiencing with 7th generation devices, so I'm thinking there might be a different explanation for why you're experiencing these problems.