Still nothing from Adobe after a FULL YEAR.
David S.,
I believe it's expected behavior with H.264 media. Premiere Pro uses the iGPU for Hardware Encoding, which speeds exporting process for H.264 files even faster than your discrete GPU in most cases. The reason? Quick Sync. Try another format, like ProRes and I bet you'll get different results.
The other case is that you will not see any discrete GPU engagement unless you are using a GPU accelerated process, like a GPU accelerated effect, scaling from 4K to 1080p, changing the frame rate, the color space, or other technical aspect of the original clip.
I think it is rather confusing, to be sure, as to which actions affect the GPU and which are more iGPU processes. Which are CPU centric and which are GPU centric? Look into Quick Sync technology and the Mercury Playback Engine's capabilities and limitations for additional information. Reading up on what's going on under the hood may help you and others in harnassing this technology to its maximum potential.
If you still feel you are not getting the right interaction with your hardware, (for example, you've added GPU effects but do not get any interaction from the discrete GPU) please let me know.
Thanks,
Kevin