This happened to me. Had my Pixel 3 charging overnight and when I woke up it was black screen and unresponsive. Plug into PC and see the Qualcomm USB device turn up, and can see it rebooting when you hold the power button. Quick search for the device shows it's stuck in Emergency DownLoad mode, due to a bootloader failure. Only ever goes back to EDL mode, so no adb or fastboot options.
3 months out of warranty. Google techsupport is worse than useless and offers to diagnose it if I ship it across the country and pay then $120+courier fees. That's a non-starter.
Retailer is being much more helpful and having it looked at on their own dime, so still waiting for the results of that.
In the meantime, Google's been shutting down bug reports on the non-beta line and just ignoring the one one linked in the article. I'd love to have a Qualcomm signed file to test out, or even access to a service centre that would try it out for me.
As a contrast, had iPhones and iPads with similar issues and the Apple Store was very helpful in diagnosing the problem at precisely zero cost to me. I may hate their walled garden, but damn they do good after-market service. Tempting to switch ecosystems.
For now, I'm making things as public as I can because I'm sure Google loves the publicity before the Pixel 6 release.