Hussar
Legend
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.Because the players assume a patch of grass is a putting green doesn't mean it is one, nor does it mean it suddenly changes height because an ambush is triggered.
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.
Again, it's the one eyed man in the land of the blind. The players cannot possibly know what they should be asking about because they are never given enough information to actually form any questions. So, again, let's not pretend that this is something that it's not. The grass is ONLY long enough to hide in because the DM needs something for the baddies to hide in to spring the ambush.