I just ran into this in 1.16.20 on HARD, so not resolved I would say...
I'm on HARD and just tried it twice. Both times, the villager got killed, not zombified. Definitely a bug. I'm on 1.16.20 on android.
I just had this happen to me on 1.16.20 on android. Carted a villager in with a zombie villager, and it got killed. No new zombie villager, just the one I already had. And just checked: I'm on hard difficulty.
And just tried it again, same result. Villager just gets killed.
Yeah but thats the whole problem
Yeah same bro
Subsequent spiders looked normal.
This was not on a new world or after an update or whatever. Just logged in again and encountered two of those.
I have the same bug also still on 1.12 on bedrock.
For me that trick solved it for like 30 seconds. A couple of mobs spawn, yes, but quickly it reverts to how it was. It looks like mobs that die actually don't get removed from the mob count or something. For me, this is still a bug, the workaround does not fix it.
I have noticed the same issue. I run a bedrock server at home that I use to play on with my PC. I usually play alone, but for a project I needed a second area loaded. However, I noticed that that second player didn't spawn anything. I built a guardian farm using nether portals to move the spawned guardians immediately.
When I am using my secondary player to linger at the farm in the overworld, nothing spawns there. Using server commands I killed everything in the ocean and then, with my main player at the kill zone in the nether, I waited. Nothing spawned. The ocean remains completely empty.
As soon as I log off the server with my main player, stuff spawns for the secondary player. I literally see a bunch of mobs spawn before my eyes the second the main player leaves. As soon as my main player logs on again, spawning for the secondary player stops.
Edited to add: version on both is 1.9.0.15
I haven't actually noticed a switch from bottom to top. It seems to me that there's a builtin preference for slabs (or maybe all blocks) to be placed in the bottom position.
I'm on android, by the way, so it's not just Windows 10.