Have you tried turning off Hardware GPU Scheduling on the virtual desktop and trying it?  Probably need to manually reboot the virtual desktop after you disable it for it to take affect.  I know in prior versions of the Nvidia drivers, that feature was only supported with GPU passthrough, not with vGPU.

If that doesn't fix it, I would try connecting to the virtual desktops from a PC with the Horizon Client for Windows installed.  See if the issue persists with PCoIP.  If it does, try switching to Blast and see if it still happens.  You can toggle on/off the H264, HEVC and Blast Codec options to see if any of them correct the issue. 

Let us know what you find.