The CauldronLevelChangeEvent is not fired when dripstone fills the cauldron with lava. For water, the event does not fire for the first level, but it does for the other levels.

Code used:

@EventHandler 
public void onCauldronFill(CauldronLevelChangeEvent e) { 
   ChangeReason cs = e.getReason(); 
   Bukkit.broadcastMessage("CauldronLevelChangeEvent: Reason=" + cs); 
}