Proof:
which one is the one I might have duplicated?
There is some evidence:
Can confirm in 1.21.7
Can confirm in 1.21.7.
The piston seems to have the hitbox that disables placing blocks through itself when it is fully extended/retracted or is being extended (expected) but it doesn’t seem to have that hitbox when it is being retracted (unexpected). If you try targeting any block through the piston, Minecraft sees you targeting that block when the piston is currently retracting.
Two of the bugs are reported there: WEB-8113 and WEB-8114