S3221QS, PIP/PBP using two HDMI ports

In the process of using the DDM (Dell Display Manager) to configure the monitors PIP/PBP dual display function, I can set the main display to HDMI1 and the sub-display to HDMI2. When I save that config, though, I get a "not compatible" error. When trying to set this up directly using the monitor's configuration buttons, it only shows the DP port choice active with the other HDMI port grayed out and not selectable. My two HDMI inputs to the monitor are my W10 PC video card (Intel GeForce GTX 745) on HDMI1 and a Google Chromecast dongle on HDMI2. Both of these work OK individually, but cannot be paired with each other as PIP/PBP HDMI inputs.

Nowhere in the manuals or specs does it says dual HDMI port selections will not work with the PIP/PBP function? I've download and installed the latest monitor driver as well as the latest DDM software, but still no joy.

So, my questions are:

1.  Is this a known limitation of the S3221QS monitor or a system bug? If the former, it should be made clear in all the docs. If the latter, it should be fixed with a Firmware/Driver update (if that is even possible).

2. In the interim, would getting an HDMI to DP converter pigtail cable to use the monitor's DP port for the second HDMI input allow this to work? Or maybe a new DP cable to connect the PC to the monitor directly?