This is an issue with both RoC, and IE. I've only played in Cathay's regions, so I don't know if it applies to all maps, or just the ones in Cathay. This applies to all units, including Skaven, Beastmen, Cathay, Vampires, and Orcs.
The terrain collision is different to the terrain visual mesh, with units sinking into it, or even floating above. The grass also floats above the visual terrain mesh.