When bonemealing a sapling with a dispenser, the StructureGrowEvent isn't called when the sapling grows into a tree.

However, if you break the tree and place a new sapling, the event gets called once you bonemeal the sapling by hand even if the tree doesn't grow. This only happens directly after a tree is grown using a Dispenser and it doesn't matter where the sapling is placed; it could be any sapling in the world.