The bug

This bug is caused by the fix of MC-141155.

When loading a structure with a structure block and set the structure integrity to a number that is between 0.0 and 1.0, the structure will either completely load or it will not load at all.

Depending on the seed, the structure will always load if the integrity is above a certain number, and it will never load if the integrity is below a certain number. Therefore, it seems like the RNG is run only once for all blocks in the structure.

To reproduce

1. Save some blocks in a structure using a structure block
2. Go somewhere else and place a new structure block
3. Put that structure block in Load mode
4. Set the integrity to some number between 0.0 and 1.0
5. Set the integrity seed to some number that is not 0
6. Load the previously saved structure
7. The structure will either be completely loaded, or not loaded at all
8. Clear the area where the structure was loaded using a /fill command, if it was loaded at all
9. Change the integrity in the loading structure block to another number
10. Repeat as often as you want

Video

Minecraft 19w12b 2019-03-21 17_31_10.mp4