PIP OFF and yet the 2nd PC extends the display

I have 2 devices connected to my monitor. When selecting one input source, I would assume that the other device does not detect the monitor, otherwise it will extend it's display to the monitor. 

I consider this a bug in the PIP code of the monitor firmware.

My setup:

MacBook connected via Thunderbolt

PC connected via DisplayPort. The PC is connected to 2nd monitor.

When I select DisplayPort (PC), the MacBook still detects the monitor and keeps extending the display to the monitor. I expect this not to be the case, but just have the internal display, when PIP is not enabled.

After contacting 1st level support, they refused to acknowledge this being a bug. Maybe somebody in here can help me. The monitor is the current flagship device Dell U4025QW

thanks for any help or for filing a request for improvement or get this in front of the firmware devs.