When you are far away from seagrass, it disappears even though the block that the seagrass stands on is very visible. You can only see seagrass from a circle with a very small radius. It is even lower than people’s render distance. And if you zoom in with spyglass it doesn’t work aswell.
It also happens to kelp but instead it will have a weird rectangle on it.
Video about the bug: https://www.youtube.com/watch?v=YYxBXx_fQSU&list=LLrYCBA4_I82iLvXCyu9zEjg