(Grabbed from https://www.spigotmc.org/threads/which-event-is-called-when-bamboo-grows.394796/#post-3545544)

When a kelp or plant grows currently, it is called using BlockSpreadEvent. However, this event is used for blocks that ""randomly" form instead of actually spread."

However, The event that should be getting called is BlockGrowEvent, because they are a "block [that] grows naturally in the world." (which is more accurate to what is happening, and will match all of its counter parts like cactus and sugarcane, etc)

JavaDoc note: These items need to be updated to them as well.