When mangrove trees attempt to spawn or grow, they attempt to extend their roots from their trunk until they hit a location they cannot be placed at. The method responsible for checking this is RootPlacer.canPlaceRoot(). The method does not return false when the requested location is outside of the world, meaning the generator is considering placing roots outside of the world, and continues along extending roots as normal. In testing, root placement is generally aborted for length reasons before properly attempting to place outside of the world. However, the fact that the generator believes it can place roots outside of the world and proceeds instead of ending roots at the world boundary is probably unintended behavior.