Probably Minecraft have bad days and don't want to load world/structures/structure.nbt/palette/(noname)/Properties/shape
Unexpected is that the shape String is loaded normally for stairs that have also this variable.
- Build some rail art as shown on Step1.png. You have to build it in the directions of the world shown in screenshot.
- Save it as a new structure using a structure block
- When you load this structure first time, you will see that it messed up the rail art as can be seen on Step2.png
- When you load it second time, it will load normally