OK, since you got me thinking about it here's a little more information that might help.
Normally I don't have a monitor plugged into the integrated graphics. That's the regimen for the above results.
In that scenario, the device manager only shows one display adpater -- the 1080Ti that has the 4k monitor connection I'm using as a primary screen.
I just now plugged in a second monitor (1920x1200, don't have a second 4k unit handy so it's possible that may be a factor(?), but see below). So now I have two display adapters listed in the Windows device manager -- the Kaby Lake HD Graphics 630 and the 1080Ti. These are listed in that order (so GPU0 and GPU1 respectively maybe??).
If I put i2pro up on the 4k display connected to the 1080Ti, the video does not show (audio is fine as before, just no images) and task manager says "GPU1 - legacy overlay" for graphics engine.
If I put i2pro up on the 1920x1200 display connected to the integrated graphics then the video plays just fine, audio and images as you'd expect, and task manager says "GPU0 - 3D" for the graphics engine.
To round out the testing, I then remove the connection from the 1080Ti and plugged the onboard graphics via displayport to the 4k display. In this situation i2pro shows the video just fine as well (audio/images no problem) and the task manager shows "GPU0 - 3D".
Hope that helps maybe!
