This is a (belated) update to this particular problem.
I just tried running kodi (v19 Matrix) on my Samsung s8 android phone. Kodi could play most x265 files that osmc Vero 4K+ can’t handle . This is with very few buffer refills, and with no stuttering. And this is over wifi to the same nfs server as with osmc. Note that I can’t run any x265 videos over wifi (from the vero). And even with hard-wired connected by gigabit ethernet, some of these still won’t play.
Of course, this does very little for me, since I really want to watch these 4K videos on my 4K UHD tv.
So it seems that this one BD video (.m2ts file; Evolution (2018)) is okay to play - at least with android.
Just wanted to update this issue.
ken