To make auto pivot work, the communication between OS and monitor is necessary.

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

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

 

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