When mirroring or rotating opened trapdoors using a structure block, they will not be mirrored or rotated correctly. This means that if you open a trapdoor in a certain direction and then mirror it it will now not be opened in the mirrored direction but instead will be opened in a completely different direction. 

Steps to recreate:

Place a structure block.

Place a trapdoor in the bounding box and open it.

Save the structure 

Load the structure and mirror it either on the x or z axis or rotate it by 90°, 180° or 270°