I know there is a few of these bugs reported already but there isn't one reported for 1.17.1 and I feel this one is a lot more descript and concise.

Essentially, when you first select LOAD of a structure with custom rotation/mirroring, the Visual Bounding box appears. Then, if you switch the mode back to Save, the bounding box keeps the rotation/mirroring of the Load Bounding Box. However, the actual blocks that are Saved in Save mode are based on the original rotation of the bounding box and not what the currently displaying Bounding box shows.

How Bug was Discovered

I discovered this when using structure blocks to transport a whole room over to another part of a building, and I rotated it to fit the new position. Since I wasn't sure if I had the block at the right height, I switched the structure block back to Save mode to quickly grab a save of the original state of the building, so I could undo the Load if it happens to be positioned incorrectly. Low and behold, it was, and so when I loaded my save that I had made of the original state of the building, it loaded another section of the building that was not too far from the structure block and that was rotated differently from where that part of the building is.

How to Reproduce

1. Place down a Structure Block
2. Open the GUI and set the mode to Load
3. Enter the name of any valid structure file (For the best result, make sure the structure file is in the shape of a rectangular prism)
4. Select the LOAD button once to display the visual bounding box
5. Select any rotation angle or mirroring
6. Switch mode to Save (Visual Bounding box keeps rotation/mirror state)
7. Name the Save something different
8. Select SAVE
9. Switch mode to Load
10. Load the newly created Save
11. It will appear as though some blocks are now missing; however, it is Saving the blocks encompassed by the original Save mode Bounding box and not the one currently displayed.