Cant cancel properly ender dragon spawn with CreatureSpawnEvent

Expected behavior

After canceling the spawn of a dragon from CreatureSpawnEvent, a bossbar should not appear since the dragon's spawn is canceled
Note: Nether portal is cancelled, only the end portal is not.

Observed/Actual behavior

After canceling the spawn of the CreatureSpawnEvent dragon, a boss bar appears, and the event is canceled (the dragon is not spawn but regardless a boss bar appears)

http://prntscr.com/121u9mc

Steps/models to reproduce

@EventHandler

protected void onDragonSpawn(CreatureSpawnEvent event) {
if (event.getEntityType() == EntityType.ENDER_DRAGON &&
event.getLocation().getWorld().getEnvironment() == Environment.THE_END) {
EnderDragon dragon = ((EnderDragon) event.getEntity());

	dragon.getBossBar().removeAll();
	dragon.getDragonBattle().generateEndPortal(false);

	event.setCancelled(true);
}

}

Plugin list

FastAsyncWorldEdit-Bukkit-1.16-655
HolographicDisplays
Multiverse-Core-4.2.2
ProtocolLib

Paper version

This server is running Paper version git-Paper-756 (MC: 1.16.5) (Implementing API version 1.16.5-R0.1-SNAPSHOT)
You are running the latest version
Previous version: git-Paper-a98d389d1 (MC: 1.16.5)

Agreements

• I am running the latest version of Paper available from https://papermc.io/downloads.
• I have searched for and ensured there isn't already an open issue regarding this.
• My version of Minecraft is supported by Paper.

Other

No response

Start by tracing the CreatureSpawnEvent handling for EnderDragon entities and the related Ender Dragon boss bar and DragonBattle end-portal logic. Reproduce the provided handler on Paper 1.16.5, then verify that canceling the end-portal dragon spawn prevents the boss bar while preserving the stated Nether-portal behavior.