Regions occasionally stop sending entities to the player

[codeHusky]

Expected behavior

Items from broken trees, as well as mobs and other entities, should be properly tracked and send to the Player's client so they can be displayed.

Observed/Actual behavior

Occasionally, regions will stop sending these entities to players, resulting in a confusing desync where you can pick up items but never see them on the game / hear them get picked up.

Steps/models to reproduce

Unsure, though this is occurring at high player counts (we've been running at 130 - 400 players)

Plugin and Datapack List

[19:21:46 INFO]: Server Plugins (28):
[19:21:46 INFO]: Paper Plugins:
[19:21:46 INFO]:  - CarbonChat, FancyHolograms
[19:21:46 INFO]: Bukkit Plugins:
[19:21:46 INFO]:  - AxInventoryRestore, BlueSlimeCore, ChangeSlots, Chunky, ChunkyBorder, CMCLinkManager, CombatLogX, CoreProtect, DatapackLoader, Essentials
[19:21:46 INFO]:  FreedomChat, GrimAC, helper, helper-sql, LiteBans, LuckPerms, NBTAPI, packetevents, PlaceholderAPI, PlasmoVoice
[19:21:46 INFO]:  PlugManX, spark, SurvivalArcade, TAB, Vault, ViaVersion

Folia version

[19:22:02 INFO]: This server is running Folia version 1.21.4-DEV-dev/adjustments-1.21.4@de75e08 (2025-01-12T21:24:37Z) (Implementing API version 1.21.4-R0.1-SNAPSHOT)
Unknown version
Previous version: 1.21.4-DEV-c84cd46 (MC: 1.21.4)

Other

This is probably a duplicate (I hope not) but figured I'd report since it seems odd. I've been able to relog and have it get fixed. There doesn't seem to be any particular pattern to the chunks that break in this way, and I don't believe my changes to the respawn logic in my fork would do anything here. If you have any vague ideas as to what could cause this kind of behavior, please lmk.

[ItsSyfe]

Just commenting here to say I've had this happen a couple times as well with player counts of ~60 up to 130 on a not too modified SMP, I've also been able to teleport into these regions and no players were visible or any entity (item frame, mobs, leads)

This server is running Folia version 1.21.4-DEV-master@8af1aef (2025-01-01T16:22:18Z) (Implementing API version 1.21.4-R0.1-SNAPSHOT)
You are running the latest version
Previous version: 1.21.4-13-4803421 (MC: 1.21.4)

[03:31:54 INFO]: Server Plugins (21):
[03:31:54 INFO]: Paper Plugins:
[03:31:54 INFO]:  - FairyChat
[03:31:54 INFO]: Bukkit Plugins:
[03:31:54 INFO]:  - AntiHealthIndicator, Chunky, ChunkyBorder, CoreProtect, DiscordSRV, Essentials, FreedomChat, GSit, JustLootIt, LuckPerms
[03:31:54 INFO]:  OreAnnouncer, packetevents, spark, squaremap, Stateful, SuperHarvest, TAB, Terra, ToolStats, voicechat

[codeHusky]

I'm a bit suspicious that Chunky may be doing something strange that's upsetting Folia somehow with how it's generating/saving these chunks on occasion. @ItsSyfe Was your world pregenerated on 1.21.4?

During my testing on 1.21.3, I had not pre generated any of the land and I don't believe anyone encountered any issues similar to this. The player counts were much lower so the regioning wasn't super active, but nevertheless I don't recall the issue occurring. It's also possible this is some sort of 1.21.4 problem in particular, but I'm unsure.

The patterns of chunks that are broken seem to be a narrow strip of chunks (maybe 2-4 chunks wide) running along the X or Z axis almost perfectly rather than random, scattered chunks, which is why I'm suspicious. The issue doesn't seem tied to the regions themselves either, which suggests to me that the chunk data is somehow "wrong" in regards to what Folia is expecting. Considering that an unload/load of the relevant chunk seems to also fix it, seems to point to chunky as well. If I could find a region that consistently behaved like this, that would be fantastic, but I have yet to reproduce this on my own

[ItsSyfe]

Yeah we pregenned with chunky but this is happening in chunks that have had a lot of player presence (it happened at a player's base for me), and unloading then reloading the chunk also seems to fix it for me as well.

Though like your case I haven't been able to reproduce it reliably as it just happens at seemingly random, I haven't been checking the chunk x y though.