You can not sneak on shulkers with {NoAI:1,Silent:1b} tags at y level 63. At any other y level you are able to sneak normally on shulkers but at y level 63 you are not able to.
A visual bug as well, if you jump then sneak onto a shulker at y level 63 then let go of the shift then the player will stay sneaking even if you are not holding shift. Then if you go into third person your character will appear to be standing normally
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We're actually already tracking this issue in MC-261073, so this ticket is being resolved and linked as a duplicate.
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