I stumbled on the lock page myself when I was experimenting with writing a sqlite vfs. It's been years since I abandoned the project so I don't recall much including why I was using the sqlitePager but I do recall the lockpage being one of the first things I found where I needed to skip sending page 262145 w/ 4096 byte pages to the pager when attempting to write the metadata for a 1TB database.

I'm surprised they didn't have any extreme tests with a lot of data that would've found this earlier. Though achieving the reliability and test coverage of sqlite is a tough task. Does make the beta label very appropriate.