NT

I play in a singleplayer world. My villager trading hall consists of several pods containing one villager each. There is a half-slab gap around the bottom of each pod for redstone, and the player is standing on a half-slab while trading. Behind each pod is a lower section (by about a half-slab or one block, I don't recall) that I use to carry a zombie in a minecart to each pod to infect villagers. 

I never pick up all of the XP orbs when trading. There are always a few orbs bouncing around in the pods after each trade. I can run around forever on the main trading floor and never pick up the remaining orbs. But if I walk behind the pods on the minecart track, I pick them all up. 

This has happened to me as well. My world is three years old, but about two years ago I mined out a slime chunk or two almost down to bedrock. It used to consistently spawn slimes in it. Yesterday I visited it again and stayed for a while, but it never spawned any slimes. I didn't spend any time diagnosing it. I mined this area out back when spawn mechanics were much different than today, but my simulation distance is 10 chunks and the distance from my player to the spawn platform falls within that range. 

This happened to me and my children as well. We all play on Windows 10. After the 1.16.100 update, when the player targets one block and beings to mine it, the game breaks two blocks (the second is behind the first) instead of just the one. Haste is not in use, but it seems like it is because the blocks break so quickly.