What I expected to happen was...:
The order of blocks staying the same
What actually happened was...:
The gravel ended up under the piston
Steps to Reproduce:
1. Build pistons as shown in the attached video with the lower one on a 2-tick repeater delay
2. Activate the lever twice in rapid secession
3. Repeat as needed
Linked issues
duplicates 1
Attachments
Comments 2
This appears to be a duplicate of MC-143508, which hasn't yet been resolved. If you have any additional information to provide regarding this issue, feel free to comment on the parent ticket. If you feel that this issue is affecting you, feel free to vote for the duplicated ticket, to give Mojang a basic idea of how many people are impacted by the problem. Just to further inform you, this will not determine how quickly the bug gets handled or fixed.
The shown behaviour was mentioned in MC-197226, although that's about piston warping; entities falling through pistons is due to 0-ticking pistons. See also videos 2+3 there.
Fixing that could lead to negative reactions by the tech community, however, if there's a way to keep desired behaviour and diminish or eradicate undesired, that'd be of course great, as also mentioned in linked bugpost.