When dragging a bridge from one elevated way to another, assuming both are flat and on the same level, this will frequently fail.
Anyways, this can be fixed by the player simply by building a bridgge of the same type somewhere on a flat ground.
Then, it's suddenly possible to build a bridge in between the elevated ways.
Thus, I assume there is something uninitialised, but I didn't check this out.
How to reproduce?
1. build two flat stretches of elevated way on the same height level.
2. Attemt to drag a bridge in between, this will (hopefully) not work (sometimes it does)
3. Build the same bridge in the same direction somewhere else.
4. Again attempt to drag a bridge in between the elevated ways. This time it will work.