This bug is caused by the randomised level of detail of each object being at 1, which causes the game to not always make it visible upon creating a scenario or a testing of a route.

Simply by going into the editor and place your objects of choice. Then double click on the object and change the "level of detail"' shown in the red circle to anything higher than 1.

This should save you from having to replace scenery and signals repeatedly on your routes.