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I think this happens because the event bus doesn't care about threads(and is static), and simply sends the event to all listeners. While you are posting the event on the server thread it goes to the event bus which iterates through all listeners, which the client-sided TEs are and gives the event to them. So in a way you made a jump from a server event to a client listener. I don't know for sure though and can only be guessing from my inspection of the EventBus class.

Well, your TileLamp only registers itself as an event subscriber on the client, so the event handler method only fires on the client, and as such the wisp is only spawned on the client.

Before the registry events fire, so in pre-init.

That is correct, but your event handler methods don't have to be static. Again, read the docs I have linked.

static initializers != static fields. Initializer means you are instantinating something. If you just use a static field to store data between methods then it's not a static initializer. This is also a false statement. Read the docs on events.

You're confusing Launcher Profiles with versions. Go into settings and select a Launcher Profile, you will be able to select what version that profile uses.