Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

The polyspellfix update a while back doesn’t seem to work with sorcerers and extra spell slots. Whenever I equip an item with a spell slot, every time I shift, I lose a spell of that level. Any way this can be fixed?

Nope and it will never work due to the manner by which sorcerers gain/lose bonus spell slots.

When a sorcerer equips a bonus spell slot item, it does not alter their spells per day until after a completed rest. When they remove a bonus spell slot item, they will still lose spells per day equivalent to the 'gained' spell per day irregardless of whether they had rested or not (try it: if you remove and equip your bonus sorcerer spell slot items eventually your spells per day will reach 0). Thus, when a sorcerer shapechanges they will always lose spells per day because the polyspellfix does not retain bonus sorcerer spell slots until after a completed rest, which a polymorphed sorcerer can never accomplish.

When a wizard equips a bonus spell slot item they get a blank spell slot which, when polymorphed, is removed again and thus no overall loss of memorised spells.