This issue is similar to #3 although now it can be easily reproduced.

Well, sometimes technology hates us and it shows.

Steps to instantly reproduce this in a singleplayer world:

1. Create a new world
2. Place a christmas tree and leave (doesn't matter if decorated or not)
3. Try to enter the world again

You will mostly likely see this screen: and you will be booted off to Multiplayer screen, the world is unusable until world/entities is removed.

On servers, the Christmas Trees just seem to despawn when player relogs, be it fully decorated or not, however there's nothing in the server logs about the trees. The drawback of this is that animals completely stop spawning on the server after that happens and the only way to get them to spawn again is to remove everything in serverworld/entities.
(seems to be also accomplished by just doing /reload, by accomplished I mean is that it effectively also kills animal spawns completely)
Client log file attached below (from singleplayer world)

MC version: 1.18
Mod Version: cspirit-1.1.3-1.18
Fabric Loader version: 0.12.8
Mods: Architectury, Christmas Spirit, Cloth Config v6, Fabric API v0.43.1+1.18, Iris, Iron Chests, Mouse Tweaks, Roughly Enough Items, Sodium, Xaero's Minimap
Server: Fabric 1.18
OS: Windows 10 x64
More visible log file: Log