I’ve been thinking about building a MOB farm and I’ve done some experimenting, so far its not looking very promising. I’ve not been able to get a MOB to spawn in or on top of a Beacon-ed plot regardless of the block type I’ve used ( natural or refined etc.)
Has anyone had any luck with any ideas so far? or better yet if a Dev reads this can you confirm the prerequisites for a MOB to spawn?
Any thoughts much appreciated 
I see MOB’s spawn when I am running through an area and all of a sudden there are 20 creatures around me ready to kill. It never happens when I am standing still. It does not matter the type of block under me. I think it has something to do with distance moved of a specific type of block spawning in an area (gleam).
Others may know better than I.
From what I understand this currently wont be possible. Mobs despawn after a certain period of time when a player leaves the area
What gave me the idea are the giant flat icebergs, if you get a good size iceberg you can run end to end and the monsters will spawn and de-spawn, top tip for hunting. The range seems to vary a little but I’d guess 5 to 10 plots away. So if you can control where they spawn like an Iceberg (mobs don’t spawn on water) then maybe you can build a mob farm.
What I can’t confirm is if they will ever spawn on plotted land, above or below plotted land or on placed blocks natural or processed.
Has this changed, not played in a few months?