Title says sand, but of course applies to falling concrete powder/falling gravel as well.

Repro:

1. Start up a save file
2. Put the attached .nbt file in .minecraft\saves\<save>\generated\minecraft\structures
3. Give yourself a structure block
4. Use the structure block to load the structure minecraft:sandbug (that you just put in the structures folder)
5. Read signs
6. Press button
7. Observe that a concrete powder breaks
8. To repeat this test, replace the concrete powder and repeat steps 6-7

Expected Result: Both columns of sand are 2 high after pressing the button
Actual Result: A sand (or concrete powder/gravel) block gets broken

A video is attached showing the issue

Observations:
1) If the stack of sand is 2 high instead of 3 high, nothing will break
2) If only one sand pile has a falling block instead of both of them, nothing will break (e.g. if you only fill one column up with sand/powder and leave the other one empty)
3) Sometimes I can quit the game and come back a couple of times and the specific sand pile I was using before won't reliably repro anymore, but in that case when I spawn in a new structure, that new structure does still reliably repro
4) Messing with the timings of the two falling sands can make the difference as to if one breaks or not (e.g. changing up the hopper flow)

5) This was not introduced in 1.16.3, I have noticed this behavior on prior versions, but am only reporting it now. I started working on a large redstone machine multiple versions ago that relies on falling concrete powder and I've never been able to get it 100% working because this bug has been around the entire time and has kept breaking my machine.