(should be 1.12+ for the version since don't think there's any big changes about structure blocks. i first discovered this bug when im working on a 1.16.1 speedrunning practice map. went testing without mods + being on the latest version: here we are.)

https://youtu.be/pGX2IXdiuZI vid link
so structure-block-structure-saving via redstone is weird. 
what i did is: 1. have a structure done built, and then place a redstone block beside the structure block in save mode.
2.  triggering load via redstone / GUI both works. (without Save&Quit)
3. after S&Q, it's no longer saved somehow, and reverts to whats before.

4. have a structure done built, and then click "SAVE" in the GUI of the structure block in save mode.
5.  triggering load via redstone / GUI both works. (without Save&Quit)
6. after S&Q, it's saved as usual.