Yeah, at a certain point the caching doesn’t do anything. There are quite a few assumptions Kodi makes and how it uses it. But that is a total different topic.
But I guess we can rule that out as you used an USB drive you know has good sustained performance.
That would suggest a problem with the codec and Kodi handling it. You already ruled out that it is a bad encode (unlikely on a remux). Guess there is a bug in h265 decoding somewhere as in “something special is not handled correctly”.
I do not know enough what components are involved here on the Vero4K. You might give a test version of Kodi 18 a try though. Or - don’t know how though - use a different player on the Vero 4K just for testing if it is Kodi related or maybe goes into some kernel module that might be involved when the file is played back.
What you could try is a quick remux with the latest mkvtoolnix, just the video and remove everything else. Just to limit it more down if it is caused by the video stream alone.
Though it would be interesting if anyone else can playback the file on a Vero 4K, just to rule out any Network / bandwidth whatever stuff for sure.
A new 4K encode would probably solve this problem for you for sure but will not get us closer to the root cause naturally.
I put my money down on something that is not properly handled by the codec at this point in regards to JW2 if other high bandwidth 4K remuxes work nicely for you.