Yes, someone (Sires) told me that this issue was solved "a few months ago",
so I kept silent and searched (read all topics) first myself before asking
* forum Problems
* https://github.com/minetest/minetest/issues
until back to Dec 2017 but found nothing.
I guess I am not able to pronounce a name for that issue, so cant find it.
So please, even after server and client are 5.1.0 and 5.0.0 still there:
Image Link
Gamer are shown 1 node to deep, walking inside the floor ...