[nevcairiel]

I would take "dual engines" more like "can work on two streams at the same time without performance loss", not necessarily "twice as fast on one stream", so I wouldn't automatically say its a clear bug, unless NVIDIA has specifically stated otherwise.

Also while of course relevant because its used on eg. YouTube, in my experience browsers are not the best choice to fully explore hardware capabilities.
Not sure if thats the test methodolgy (like not excluding the first second of startup glitches), but not even able to produce fully fluid 4K 60 fps is either a software or a testing failure, because a dedicated video player certainly can render 4k 60 fps without any frame drops.