The game will revert a structure block save file state to it's previous save upon relogging. It's also completely random whether it will happen or not.

(Recorded video file exceeds the sites limit, so I've provided a link to a YouTube video that showcases the bug) 
https://youtu.be/v1l1GN5knwM?si=tA86ky9CUnvZ5yc5
https://youtu.be/vsyZkRYLXhg

Steps to reproduce:

1. Create a creative mode world with cheats enabled and give yourself a structure block.
2. Build and save a structure with that structure block, what you save doesn't matter. 
3. Use another structure block to load the saved structure in a different location. 
4. Change the original structure in a noticeable way, and then save it by powering the save block with redstone.
5. Leave and rejoin the world.
6. Power the load structure block with a button.
7. Power the save block with the button again, then the load block.

Observed Results:
The loaded structure will be in its first state upon reloading, but upon powering the save block again, it will update and load the more recent saved version of the structure.

Expected Results:
After reloading the world, the structure should have immediately loaded the correct and updated version.

Note: 
This does not happen when the structure is saved via the structure block GUI.