Piglins with the persistence tag set to true after they have spawned (for example, naming piglins, or piglins that have picked up golden shovels) count towards the mob cap, which is not vanilla behavior.

This has the effect that a lot of nametagged mobs, for example, can stop mobs from spawning when loaded by filling the mob cap. This affected my barter farm (which stops all other nether farms from working due to the piglins being loaded, which wouldn't happen in vanilla) and my wither skeleton farm (which uses nametagged piglins, so rates are much lower in spigot than vanilla).

Server config: spigot default

Tested with fresh server, no plugins

Steps to reproduce:

1. Go to nether roof, make a pen and fill with 70+ piglins using spawn eggs. You can either name tag every one of them, or drop a bunch of golden shovels so they all pick them up. (Optional: verify they have the persistence tag with /data)
2. Fly up 130 blocks so all mobs below nether roof despawn, as well as any piglins that aren't persistent
3. Go below nether roof under where the piglins are, no mobs spawn

You can further verify by:

1. Chunk load the piglin pen with either a portal chunk loader, or /forceload
2. Teleport 1000 blocks away, no mobs should spawn
3. Do `/kill @e[type=piglin]`
4. Mobs will resume spawning normally

EDIT: this issue only affects piglins, thanks Airtheon for testing