[Yappa]

if that doesnt fix it hop over to the MADVR forum and report it there, there have been a number of driver related issues recently with NVIDIA cards but also an issue with render queues not filling up but I think that was fixed on . MADVR .17, are you up to date on both MADVR and LAV filters, remembering that you have to update the LAV filters manually in the KODI install folder.

fulllscreen exclusive mode is the only way I've heard of to get smooth playback. Make sure the OSD says "exclusive." 

Reinstalling helped, resulting in smooth 4K HDR playback and my GPU usage is now low enough that the graphics card stays silent as it should be.

My current problem though is that my LG C7 OLED will now switch to HDR mode for all video material, no matter whether it's HDR or SDR. Only the Kodi menus without any movie playing in the background are displayed in SDR. Doesn't matter whether I use the default Kodi VideoPlayer or DSPlayer for the SDR video.
Currently using Nvidia drivers 411.70 and went with RGB full range, 8bpc in the Nvidia settings.
Guess, I'll need to get an older driver that does a better job handling autoswitching. Problem is that my new card is probably not supported by versions such as 376.33.

Edit:

Only happens when using hardware acceleration/DXVA2, so I could disable it, though that shouldn't be necessary...