Thank you for your report! We are aware of this issue. The reason why these props are floating is because in GTA: III and GTA: VC, these props are not placed correctly in the files. The height is calculated at runtime. In GTA: SA and GTA: UG, this logic is not present. Now, without any tweaks, this would result in a lot of lampposts (including the one which is floating in your screenshot) being sunk into the ground, as well as a lot of height-related positioning issues.
This is something that we (poorly!!) resolved by correcting the positions in the files by hand, however, as you reported, we made numerous mistakes. To make a long explanation short and to get to the point, we are going to fix all of these misplacement issues in one go very soon. Perhaps in one of the updates for Snapshot 3.4. With that said, I'm marking this entry as Verified and we'll update your report once the issue is resolved.