EntityAddToWorldEvent behaves differently after ChunkSystem rewrite

[aerulion]

Expected behavior

Everything should be behaving the same as before the patch.

Observed/Actual behavior

Mainly I came across three things, which are probably all related to each other:

1. Entities can't be removed during the event anymore. That used to work just fine. Calling entity.remove() will lead to the following error: https://pastebin.com/raw/HBWfmAyH
2. The content of item frames can't be updated during the event, which leads to a similar error: https://pastebin.com/raw/53GUDLay
3. The third thing I'm currently unable to reproduce on a clean test server, so it's probably not relevant, but I have the suspicion that the event is called at a different point of time during startup, because on our production server we were able to process entities added in the spawn chunks (even without "load: STARTUP" plugin.yml setting, due to other plugin depends), which stopped working after the update.

Steps/models to reproduce

Listen to the EntityAddToWorldEvent and removing the entity, or modifying the content of an item frame.

Plugin and Datapack List

Just a single test plugin.

Paper version

This server is running Paper version git-Paper-177 (MC: 1.19.2) (Implementing API version 1.19.2-R0.1-SNAPSHOT) (Git: 1bbd273) You are running the latest version

Other

These are probably all things that can be fixed by scheduling the removal or content update a few ticks later, but I still expected the event to behave as before the update.