issue : x265 / HEVC – video playbac shows stair-step / zig zag effect

I just unwrapped a PC I built for my mum as a Crimbo present and ended up scratching my head quite a bit so I thought I’d better provide feedback.

Build is an intel skylake pentium G4400, no extra graphics card. So basically decoding is handled by the CPU alone
Other details include 8GB of memory & Gigabyte B150N motherboard + M2 SSD running on Windows 10 Pro (N+Win media pack)

I’m getting “visual effects” during playback, but only on files I’ve encoded using handbrake (HEVC, 720p or 1080p, constant quality 25, all else default)
this happens on Jarvis and the latest Krypton beta
best way to describe this is : picture for the most shows all details normally, however, there’s a kind of mosaic effect.
essentially there are two lines of super large squares showing, these squares seem to have slightly different colours making them noticeable.
It’s like the squares are “translucent”, but are set at different percentages.
These squares are arranged diagonally like a set of stairs

I thought maybe I’d done something stoopid with my encode so I got my hands on some less than legal sources (TV series etc.. from PB)
Same issue, so it’s not my encodes.
I then tried x264, no issues whatso-ever
I then tried VLC, no issues whatso-ever
I then tried MPC-HC, no issues whatso-ever
All files play just fine on my own HTPC using Kodi
…and I do not remember seeing anything on a really old Pentium PC I installed in the kitchen (this has a very old dedicated graphics card)
conclusion. Perhaps skylake pentiums not fully supported by Kodi if using the onboard graphics??

(or is this because I mistakenly installed Win 10 “N” and had to then install Windows Media pack seperately? I know I couldn’t play any music until I did this so I have to wonder if maybe something is still missing)

If this has been reported elsewhere I apologise I did run several searches and didn’t find anything (i.e. I hope this is useful)

Merry xmas to one and all