I'm seeing the same behavior.  Auto-rotate stopped working when the update was retrieved (and I got a message to restart for update to kick in.)  I've used it since buying this Chromebook almost two years ago.

I'm no longer seeing auto-rotate as an option. And the display doesn't rotate, and the keyboard doesn't dis-engage when I flip the screen around into tablet mode.

Chrome OS Version 70.0.3538.76 (Official Build) (64-bit)

Acer Model N15Q8

Re#0, #1: It sounds like the device stays in clamshell mode even it's flipped over into tablet position. Did you have an external mouse (or other external pointing device) attached? Currently, an external mouse will make the device stay in clamshell mode. 

I have no external device connected at all to my machine.  I also have no bluetooth device connected.  I have recently tried a powerwash which did not resolve this issue.

The external mouse appears to have been the issue for me.  I did not have a mouse attached physically, but I saw that Bluetooth was on and a mouse in the room had previously been used with it.  I turned off Bluetooth, twisted the screen into tablet mode and rotated it and found that it works in tablet mode as expected. 

Thanks for pointing that out. I wonder if some message couldn't be added when a user tries to put it in tablet mode but tablet mode is disabled because of an attached mouse.  Feature request, I suppose. 

dave@, thanks for the suggestion! 
imunclebubba@: can you check if your device is in clamshell mode? You should be able to tell if your device is in clamshell mode from 1) there is no back button on the left corner of the shelf 2) browser window has its controlled buttons (minimize/maximize/close button) 

Per #6， mccanny@, omrilio@, what do you think about popping up a notification for the case that when the user flip a device into tablet position or detach a device from its keyboard base (which clearly states the user's intention to enter tablet mode), but an external pointing device prevents it entering tablet mode?

I am in clamshell mode.  I have no back button and the browser has the minimize/maximize/close buttons visible.  I can't get out of this mode.

Re imunclebubba@: That explains why auto-rotation didn't work. Currently auto-rotation only works when the device is in tablet mode. 

If you don't have any external pointing device (usb or bluetooth mouse, touchpad) and you can't get out of this clamshell mode by flipping the lid back, Chrome may not receive any accelerometer data at all, which might indicate there is some problem with hardware?

Alright. I'm currently on beta channel, it just had an update and when I restarted the device is allowing me to go into tablet mode and tent mode. It is functioning as it is supposed to as of right now.  Will keep track of device and see if this maintains. If it doesn't I will see if there is a hardware issue.

Sg. I'll close this issue then. Feel free to reopen it if you can still see the same issue.

Less than 24 hours later and the issue has returned.  With it being sporadic in nature it's hard for me to diagnose if it is a software bug or a hardware issue.