Yes they do. Car panels too. For the bridge pieces over the water this means the loot is lost forever and for the ones over grass means the loot despawns before you can get to it though.
-Building placement is quite frustrating at times. Harvesting corn that was planted behind a tower is also quite the undertaking.
Is this a visual problem? I'll try it out and see whats going on with that
The upgraded towers are very high. When corn is behind that you can't see multiple rows so harvesting becomes hitting blindly all around you in the hopes you get everything before you replant.
For building placement, it seems to favour placing it next to something already there, making it hard to place something in a void when other structures are around that void.