Redstone wire, powered rails, activator rails, detector rails and vines that are placed on top of a tile entity block (furnace, command block, dropper, etc.) in a structure file will break when that structure is loaded if the block that was below prior to the structure being loaded could not have supported them.

To reproduce:
Save a two-block structure: a dropper (or similar block) with redstone dust on top of it, then load that structure in empty air. The redstone will break and become an item.

Reload the structure in the same location, and the redstone will not break, because this time there was already a block positioned below it that could support it.

Try again with a regular block instead of a dropper and the structure will load normally in both cases.

I assume this happens because tile entity blocks are handled differently than regular blocks when loading a structure, but I have no idea why this happens with redstone wire and special rails but not regular rails or other blocks that require support like carpets, torches, or repeaters.