Shulker can be pushed to outside the world border by the piston.
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Can confirm in 20w48a.
I am unable to reproduce this issue in 21w03a.

Behavior is bit different in 21w03a, but can still be reproduced there.
Can still confirm in 21w05a despite behavior change.
Can confirm in 21w05b.
Can confirm in 21w06a.
Video attached.
Can confirm in 21w07a.
Can't reproduce in the 1.17-pre2 snapshot.