The reason is SCS component Potions for NPCs having this code
LAF clone_item STR_VAR item=~misc02=>dw#haspt~ editstring=~droppable=>0~ END
It creates a dummy item from an existing file Misc02.itm = mirror.
SCS installs very late in the sequence of mods and some earlier mods can apply changes to the item before.
For example, the reflect gaze effect for mirror is added by either Aurora mod or Deepgnomes mod if you have one of those installed. These mods allow you to use the mirror in the shield slot. But that added effect just makes the flaw in the SCS code visible, the source is SCS not using a unique item but a clone.
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