When you fill your inventory and have a shulker box on the ground, it is added to your inventory and disappears
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We're tracking this issue in MC-8409, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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If in creative mode, tis is intentional and not a bug.
Can confirm. This happens with any item that is not able to be placed in the inventory, not only shulker boxes. Also only happens in creative mode.