The only place I'm aware of this problem is on Azteca Lite, where the INF vehicle meshes (5 ton truck & hummers) you see offline don't show up online, but the collisions are still there. This happens because the map uses invisible collision brushes for the meshes, instead of the standard UT collision cylinders. Works great offline, as it gives you more precise collisions, making it feel like the meshes are actually BSP brushes. But unfortunately UT online code makes it think the meshes aren't visible to players, so they aren't drawn for clients. So w/ 2.87 I'm releasing a "fixed" version of Azteca Lite where I go back to the non-brush collision cylinders, and you'll see the vehicles online again.
If you're seeing this problem on other maps, please say which, so we can look into those as well.