No. It absolutely does suddenly change height. You describe it as some grass over there and nothing else. That means that the players have a very incomplete view of the scene. That the grass might be long enough for someone to hide in is absolutely something that adventurers traveling through dangerous wilderness would immediately note. Heck, that's part and parcel of basic training.
But, you just describe it as "grass". So, the players don't pay any attention to it because if there was any reason to pay attention to it, you should have told them so.
But, you don't. Dm's never do.