I have the same problem as well, I deal with it in mainly two ways:

1. I use the Ctrl-Alt-arrow key to manually force the screen orientation.

or

2. I hit the sleep button and immediately wake up the device and the device will start to auto rotate.

I know it's supposed to work better than this but this is a quick and simple solution for now.

My screen stopped rotating after some support.dell.com updates. I restored an earlier backup of my 9250, and rather updating through Dell I updated my video, audio, and USB drivers by using Windows 10 device manager and searching automatically for updated driver software. Now my screen auto-rotates again, asking to switch to tablet mode. Went through most every device in device manager this way, most saying "the best driver software for your device is already installed" but several devices did get updated drivers doing it this way. I did update to BIOS version 1.1.25 through Dell previously.