To make auto pivot work, it's necessary to communicate between OS and monitor.

In Mac OS X or prior version, such communication is done by HDMI signal.

However, in Mac OS Sierra, Apple changed the behavior in which this is no longer communicated with HDMI signal.

As a result, it's necessary to connect both VP2468/VP2768 and iMac by a "type A to type B" USB cable as the followings.