The bushes are whats known in the engine as "Soft Cover".. they basically have collision for weapons but not anything else. The grass vegetation on the map is a crytek model, so we have no access to the source to change it. I belive the actual material itself, which is a custom one made by the MWLL team, may be part of the problem too..
These issues are both something we will look at though, as we get more of our own environmental assets created to replace Crytek's.
Here is a screenshot attached of the editor with TC_Marshes open, displaying the "Soft Cover" hitboxes on the Grass and bushes in a small area(which are completely different than a collision hitbox btw, you can see both in this image)...
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