In the latest version of Minecraft Java Edition only, Grass does not act the way it used to and how it acts on the Bedrock Edition of the game. Grass and Mycelium will not suffocate when underwater and will spread to dirt nearby, this could be a problem to not only people with slower computers but on servers, if a big server has many chunks loaded, the grass and mycelium could cause some lag on servers. The fact that it only happens on the Java Edition leaves me to believe that this was unintentional.