Hello!~
So I've noticed quite a few posts and or previous people mentioning that their farms aren't working as intended. That the range of spawns is nerfed or tweaked.
So I have finally figured out what is going on here. And I am going to post this information in this post here so that way others can find it and hopefully it helps them figure out what is going on or a solution to fix it.
First I will start this post off with the following. I created a Creeper farm that used a vertical AFK position over a deep ocean to farm creepers. I noticed my haul was virtually nothing. Upon further testing, by using my main account and a alt to stand at the AFK location then teleporting to the kill area of the farm. I could confirm that the creepers were spawning normally.
I attempted to use spectate mode while someone was in the AFK spot to watch the farm in action, and I notioed it was working completely fine when I was nearby in spectate and I had someone in the AFK spot. Odd.
So I tested again by having a solo in the afk spot and the haul again was nothing.
Even more odd.
Cranking render distance and every range render thing to max and using Xray + glass. I was able to to confirm that mobs de-render at X range, but this doesn't stop them from existing.
What I learned is that snowmen and potentially general MOB AI seems to just stop working when you are 30 blocks away from them in Y height.
As for what is creating this issue, unsure if it is the modpack or vanilla 1.20.1 gameplay.