Documentation of World.getHighestBlockYAt says:

Gets the highest non-air coordinate at the given coordinates

But it actually returns Y of the block above. It also ignores trasparent block (opacity == 0). So the result is that if the top block is tall grass or snow it correctly returns Y of the top block, but if it's not - it returns Y of the block above.

It also causes world.getHighestBlockAt to return incorrect block.

This behavior has been there for a long time and all plugins using these methods probably already rely on the wrong behavior (at least my plugin does and it would break if the behavior changed).