Carnivorous cubicle summons the nerfed version of Nythandra instead of the full statted one.
Each sub-version of Nythendra is its own dedicated card, not a nerfed version of Nythendra
You can see that by hovering the minion in play
A 5/5 Nythendra, when hovered, will display a 5 Mana 5/5 Nythendra, not a 7 mana 7/7 that would have been nerfed
Since each statline has its own version, destroying it and summoning a copy of it, a plain copy of the destroyed version, will correctly summoned the reduced version and not the original one, being a different card