An Intellect Devourer Has an ability called Body Thief, which states:
...the intellect devourer magically consumes the target’s brain, teleports into the target’s skull, and takes control of the target’s body. While inside a creature, the intellect devourer has total cover against attacks and other effects originating outside its host. The intellect devourer retains its Intelligence, Wisdom, and Charisma scores, as well as its understanding of Deep Speech, its telepathy, and its traits. It otherwise adopts the target’s statistics. It knows everything the creature knew, including spells and languages.
The simulacrum lacks the ability to learn or become more powerful, so it never increases its level or other abilities, nor can it regain expended spell slots.
This literally just happened to us and we are stumped. I think the Devourer is a new creature and thus can recover spells. As it would choose to gain the abilities but not the short comings of the new form.