Thanks for the overview. I know HN isn't the place for this but just for the record: there are no other i915 messages in dmesg (certainly nothing about a file with debug info).
This is on Ubuntu 19.10, kernel 5.3.0, and it started happening after a minor update, so I figured it would probably get fixed in another minor update. That didn't happen, so then I thought I'd better update to a newer kernel before reporting anything. Then 20.04 was almost here, so I decided to wait for that. Hopefully I'll get around to that soon and discover my particular bug has been fixed, otherwise I'll try to report it.