Filed from user feedback:

For the past few days, on ChromeOS beta: on a Pixelbook connected to a larger screen, upon wakeup/login, the default screen is back on the laptop vs the large screen.  I.e.,
I work on the large screen, lots of tabs open.
Walk away, Pixelbook goes to sleep.
Come back, wake up laptop, login.
Bug: big screen is empty (background image), the tabs that were open are now open on the laptop screen.
Collapsing the window and then opening it again moves it to the big screen.  The video below shows how it looks.

https://photos.app.goo.gl/j51NFDoFTfXanHY59

Might be too late for m66. Let me find if I can dig out what is wrong here and try to catch m67.