h.264 h/w encoding

Hello forum folks,

I've had encoding issues since upgrading to High Sierra / CC2018. on a MacPro 5,1, dual 3.46 6core , 48GB, GTX980ti 6GB, OS 10.13.5.

Had almost no issues on Yosemite / CC2017.

Now when encoding to h.264. If I encode to 720 I get hardware encoding. If I encode the same file to 1080 I only get software encoding.

I've just selected the Vimeo or Youtube presets for either frame size, it doesn't matter.

Why would 1080 cause this behaviour?

I can repeat this on other similar machines.

I've been reading on other forums folks having all sorts of problems encoding and Adobe is saying that the hardware is not compatible.

Obviously the hardware is compatible If 720 works perfectly, why cripple the 1080?

Bruce