The bug
Arrows bounce off the shulker as long as the shulker has started closing. This means that the split second they start closing, arrows will bounce off despite them still being almost completely open.
Additional information
It is suspected that this affects melee attacks too; however, this has not been confirmed.
Linked issues
Attachments
Comments

Is this an issue in 1.16.5?

Can confirm on 1.18 pre1.

Can confirm on 22w11a.
Can confirm in 21w15a.