Steps to Reproduce:
Create world any.
Place two pistons (above the sticky piston put the usual)
Connect to sticky piston repeator (Volume 2)
Above repetor set any block
Put readstone dust on setted block
Сonnect the entire circuit
Set on basic piston 2 falling blocks (For example: "Sand")
Power the circuit from the button
Wait
Power the circuit from the button
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.
As already written in the still-open bugpost MC-198112 ("Gravel goes through piston"):
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.