If pick block is used to draw an item from a shulker box in the player's inventory shortly after a tool from the toolbelt has been used (and is thus considered to be in the player's hand), the item is placed in the hotbar as intended, but it's place in the shulker box is then taken by a perfect duplicate of the tool from the toolbelt. This duplicate is fully functional (i.e. does not disappear if moved in the inventory, used to mine a block or anything else that would cause fake items to delete themselves).

Repro-Steps:
Stand in near a block that you also have a stack of in a shulker box within your inventory). Perform any action that would cause a tool from the toolbelt to be selected, then use pick block.

Items that are in your normal inventory are not affected.