Retracting pistons do not have any hitbox, for a slight moment. Normally this would not be a big problem. I think it is related to MC-54546 and MC-81651 and MC-93631. In the screenshot (btw, it was quite hard to take this screenshot because it's instant), the piston hitbox disappeared, and the stone's hitbox appeared, which is behind the piston.
Linked issues
discovered while testing 1
is duplicated by 4
MC-169053
Pistons are hard to remove when activated fast. Blocks below or behind will probably get destroyed earlier before the piston does.
Resolved
MC-176994
Piston Targeting
Resolved
MC-193920
You can target through extending/retracting pistons
Resolved
MC-199556
Blocks can be placed on the other side of rapidly extending pistons.
Resolved
relates to 2
Attachments
Comments 19
This is because for a split second the piston turns into the block entity moving_piston which has no hitbox. A solution for this is to give moving_piston a block outline.
Can confirm in 21w03a. This can cause you to break blocks through the piston whilst it's retracting.
9 more comments
This appears to be the cause of MC-93631 as any block that requires a block to be under it breaks when the piston retracts.