I played around with pistions and everytime it extended or contracted, it seems an added hit box appered.
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not understanding your bug. That hit box in the screen shot is the piston body's hit box. when the piston extends it gets two hit boxes, the body and the arm. This is intended.

Duplicate of MC-1677, please use the search function to see if your bug has already been submitted. Currently over 57% of tickets are being resolved as duplicate.